%%* "A Completeness Theorem for TLA"
%%* Long hand proof, unchecked (Oct 90 version)
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\begin{document}
\begin{spec}
A COMPLETENESS THEOREM FOR TLA
Preface: A Quick Introduction to TLA
-------------------------------------
Here's a quick and dirty informal definition and semantics for TLA.
A more precise one is given later. If you are already familiar
with the new, improved TLA (defined a little differently than in
SRC Report No. 57) you can skip to the Introduction.
Values:
I assume a set of values, big enough to contain all the constants
of interest. It includes the values 1, TRUE, NAT (the set of all
naturals), {n :in: NAT : n a prime}, etc.
State, Variable:
A variable is something that assigns a value to every state. I let
s.x, denote the value state s assigns to variable s. Or maybe a
state s is something that assigns a value s.x to a variable x.
Take your pick.
State Function:
An expression made from variables and constants, such as x^2 + 3*y.
A state function f assigns a value s.f to every state s. For
example,
s.(x^2 +3*y) = (s.x)^2 + 3*(s.y).
Predicate:
A boolean-valued state function--for example,
x^2 < 3*y
Action:
A Boolean expression involving variables, primed variables and
constants--for example, x + 1 < 2*y'. An action maps pairs of
states to Booleans. Letting s.A.t denote the value that action A
assigns to the pair (s,t), I define
s.(x + 1 < 2*y').t = (s.x) + 1 < 2*(t.y)
In other words, the unprimed variables talk about the left-hand
state, and the primed variables talk about the right-hand state.
Think of s.A.t = TRUE as meaning that an A-step can take state s to
state t. An action is valid, written |= A, iff s.A.t is true for
all states s and t.
Enabled(A):
For any action A, the predicate Enabled(A) is defined by
s.Enabled(A) == :E: t : s.A.t
f'=f:
For any state function f, the action f'=f, which is sometimes
written Unchanged(f), is defined by
s.(f'=f).t == (t.f) = (s.f)
[A]_f:
The action [A]_f is defined by
[A]_f == A \/ (f'=f)
An [A]_f step is either an A step or leaves f unchanged.
_f:
The action _f is defined by
_f == ~[~A]_f
It equals A /\ (f'#f). An _f step is an A step that changes f.
The Raw Logic:
A Raw Logic formula is any formula made from actions using
logical operators and the unary [] operator--for example
A \/ [](B /\ []~[]~A)
where A and B are actions. A Raw Logic formula is a Boolean-valued
function on infinite sequences of states. An infinite sequence of
states is called a BEHAVIOR. An action A is interpreted as the
temporal formula asserting that first step of the behavior is an A
step. The formula []A asserts that every step is an A step. In
general, let s_0, s_1, ... |= F denote the value that formula F
assigns to the sequence s_0, s_1, .... The semantics of Raw Logic
formulas is defined as follows, where A is any action and F and G
are any formulas:
s_0, s_1, s_2, ... |= A == s_0.A.s_1
s_0, s_1, s_2, ... |= []F
== :A: n :geq: 0 : s_n, s_[n+1], s_[n+2], ... |= F
s_0, s_1, s_2, ... |= ~F == ~(s_0, s_1, s_2, ... |= F)
s_0, s_1, s_2, ... |= F \/ G == etc.
A formula F is valid, written |= F, iff it is true for all
behaviors.
TLA:
The Raw Logic is wonderfully simple, but it is too expressive. It
allows you to assert that something is true of the next state,
which ruins any effort to heierarchically refine programs. We
define TLA to be the subset of Raw Logic formulas obtained by
application of [] and logical operators starting not from arbitrary
actions, but from predicates and actions of the form [A]_f. For
example:
P => ~[]~[][A]_f \/ [](Q => [][B]_g)
Observe that [][A]_f asserts that every step is either an A step
or else leaves f unchanged.
As is usual in temporal logic, we define <> and ~> by
<>F == ~[]~F
F ~> G == [](F => []G)
The Raw formula <>A is a TLA formulas iff A is a predicate or an
action of the form <>_f.
Technical point. Since []F /\ []G = [](F /\ G) holds for any F and
G, it's convenient to let TLA include formulas of the form
[](P /\ [][A]_f) where P is a predicate.
Introduction
------------
This is a relative completeness proof for TLA, a la Cook. It is
not a completeness result for all of TLA, just for the class of
formulas that one is interested in proving. The formulas we're
interested in are of the form
Program => Property
A Program has the form
P /\ [][A]_f /\ Fairness
for P a predicate. So far, all the Fairness conditions have
have been conjunctions of the form SF_f(B) or WF_f(B), where
B implies A and
WF_f(B) == []<>**_f \/ []<>~Enabled(****_f)
SF_f(B) == []<>****_f \/ <>[]~Enabled(****_f)
The theorem allows the more general class of programs in which
Fairness is the conjunction of formulas of the form
[]<>~TACT \/ <>[]TACT or []<>~TACT \/ []<>TACT,
where TACT denotes any formula of the form Q /\ [B]_g, so ~TACT is
a formula of the form Q \/ ****_g. The Fairness formula must
satisfy the additional requirement that program is machine closed,
meaning that for any safety property S:
If |= (P /\ [][A]_f /\ Fairness) => S
then |= (P /\ [][A]_f) => S
(The theorem requires this only when S is of the form []TACT.)
Machine closure, which was called "feasibility" by Apt, Francez,
and Katz, is a reasonable requirement for any fairness condition.
It can be argued that a condition not satisfying it is not a
fairness condition, since it can't be implemented by a memory-less
scheduler.
The Property can have any of the following forms:
Predicate
[]Predicate
Predicate ~> Predicate
GeneralProgram
where a GeneralProgram is like a Program, except without the
machine-closure requirement on its fairness condition. The absence
of this requirement is important, for the following reason. To
prove that program :Pi:_1 implements program :Pi:_2, one proves
:Pi:_1 => :Phi:_2, where :Phi:_2 is obtained from :Pi:_2 by
substituting state functions for variables. This substitution
preserves the form of the formula :Pi:_2, but can destroy
machine-closure.
Proving relative completeness for safety properties in TLA is
pretty much the same as proving it for the Floyd/Hoare method. The
completeness results for Hoare's method assumes the expressibility
of the predicate sp(S, P) for program statements S and predicates
P, where sp is the strongest postcondition operator. Assuming such
predicates for arbitrary statements S, which include loops or
recursion, is equivalent to assuming the expressibility of sp(A, P)
and sin(A, P) for atomic actions A, where sin is the strongest
invariant operator.
Proving relative completeness for liveness is somewhat trickier.
It requires induction over well-founded sets. Taking a simple,
intuitive approach leads to a result whose practical interest is
rather doubtful. For example, Mann and Pnueli ("Completing the
Temporal Picture") use the axiom of choice to pull a well-founded
ordering on the state space out of a hat. Such a construction
requires the assumption that every semantic predicate is
syntactically expressible.
Getting the precise expressibility assumptions, and avoiding
mistakes, required a careful formal exposition.
The Assumptions
---------------
In relative completeness results for Hoare logic, one assumes a
complete system for reasoning about predicates. In TLA, all the
serious reasoning is in the domain of actions. So, we assume a
complete system for reasoning about actions. More precisely,
letting |- denote provability, we assume a set ACT of expressible
actions such that (|= A) => (|- A) for any action A in ACT. There
are various simple assumptions about ACT--such as its being closed
under boolean operations. Let PRED denote the set consisting of
all predicates in ACT (remember that a predicate is an action that
doesn't mention unprimed variables). The least reasonable
assumption is that for any P in PRED and A in ACT, sin(A, P) and
sp(A, P) are in PRED. Of course, this assumption is what really
puts the "relative" in "relative completeness".
The relatively complete logical system consists of the following:
1. The usual assortment of simple propositional temporal logic
rules and axioms that you'd expect, since TLA includes
simple temporal logic (the logic that's the same as the Raw
Logic except starting with predicates, not arbitrary actions).
2. An induction principle, which is what you'd expect for any
relatively complete system for proving temporal logic
liveness properties.
3. The two TLA axioms:
|- ([]P :equiv: P /\ [][P => P']_[P])
|-(A => B) => |-([]A => []B)
where P is a predicate, and A and B are actions of the form
P /\ [A]_f.
The axioms of 3 are the only ones that mention actions. The axioms
of 1 only mention arbitrary formulas, and the induction principle
of 2 talks only about predicates. These axioms are actually valid
for the Raw logic, and in that logic the second axiom of 3
is a special case of the axiom
|- (F => G) => |- ([]F => []G)
from 1, for arbitrary formulas F and G. However, [A]_f => [B]_g
isn't a TLA formula (it's a formula in the logic of actions, but
not in TLA), so the second axiom of 3 is needed if you want to do
all your reasoning completely within TLA.
The induction axiom 2 is tricky enough to be worth mentioning. To
get it right, we first have to generalize everything to include
logical variables. If you want to describe an n-process algorithm
with a TLA formula, for an arbitrary but fixed n, then n is a
logical variable of the formula. A logical variable represents an
unspecified value that is the same for all states of a behavior.
In the semantics of actions and TLA formulas, Booleans have to be
replaced by Boolean-valued formulas involving logical variables.
(Formally, Booleans become boolean-valued functions on
interpretations, where an interpretation is an assignment of values
to all logical variables.) Logical variables pop up all the time
when you use TLA in practice. For example, if you have a
distributed algorithm with a set Node of nodes, then Node is a
logical variable. In fact, if you go really overboard in
formalism--as you must to verify things mechanically--then
everything that's not a program variable (the kind of variable I
first talked about) or a logical operator is a logical variable.
In the expression x + 3, the + and the 3 are logical variables. We
just happen to have a lot of axioms about the logical variables +
and 3, such as 1+1+1 = 3, while we have just a few axioms about the
logical variable n (for example n :in: NAT, n > 0).
But, I digress. I was talking about the induction principle. An
induction principle involves induction over a well-founded ordering
on a set. Intuitively, a well-founded ordering on a set S is a
relation > such that there does not exist an infinite sequence
c_1 > c_2 > c_3 > ... . More precisely,
Well-Founded(>, S)
== ~ :A: i : (i :in: NAT) =>
:E: c_i : (c_i :in: S) /\ (c_i > c_[i+1])
But, what does this formula mean? For me, the most sensible way to
interpret it as a logical formula is to rewrite it as
Well-Founded(v > w, S)
== :A: c : ~ :A: i : (i :in: NAT) =>
(c(i) :in: S) /\ (c(i) > c(i+1))
where v > w is a formula with free logical variables v and w, and
(c(i) > ci+1)) is the formula obtained by substituting c(i) for v
and c(i+1) for w in the formula v > w. This is a higher-order
formula, involving quantification over a function symbol c.
The completeness result requires, as an assumption, that if the
formulas "v > w" and "v :in: S" are expressible, then
Well-Founded(v > w, S) is provable if it's true. I think that if
you look closely at Manna and Pnueli's paper, you'll find that they
are implicitly assuming this for any formula "v > w"--not just for
an expressible one.
Anyway, the actual temporal induction principle looks as follows,
where P(w) denotes a formula containing w as free logical
variables, P(v) denotes the result of substituting v for w, and F
is an arbitrary temporal formula.
If |- :E: w :in: S
w not free in F
|- Well-Founded(>, S)
|- (F /\ w :in: S
=> (P(w) ~> :E: v : (v :in: S) /\ (w > v) /\ P(v)))
then |- ~F
I've actually lied a bit. I assume this rule when w is a k-tuple
of distinct logical variables, and I assume the provability only of
Well-Founded(v > w, VAL^k), where v > w is an expressible formula
and VAL^k denotes the set of k-tuples of values. I could have done
it the other way by making a few more expressibility
assumptions--such as assuming that "v :in: VAL^k" is
expressible--but I think that would have been a little more
complicated.
ACTIONS
-------
Primitives
-----------
The following are primitive notions, along with their intuitive
explanations.
VAL : A set of values, containing the values TRUE and FALSE
(among many others). The semantics of TLA is based
on this set.
ST : A set of states.
PVBL : A set of program variables. These variables appear in TLA
formulas and represent the primitive state components.
That is, a state assigns a value in VAL to every variable
in PVBL.
LVBL : An infinite set of logical variables. A logical variable
denotes a fixed, unspecified elements of VAL; it represents
a program "constant".
ACT : The set of expressible actions.
PRED : The set of expressible predicates.
SFCN : The set of expressible state functions.
Notations
---------
VAL^k == The set of all k-tuples of elements in VAL.
PVBL^k == The set of k-tuples of DISTINCT program variables in PVBL.
LVBL^k == The set of k-tuples of DISTINCT logical variables in LVBL.
Two k-tuples v, w :in: LVBL^k are said to be DISJOINT iff they
have no components in common.
For v :in: LVBL^k, f any formula, c in LVBL^k or VAL^k:
f[c/v] == The result of substituting each component of
c for the corresponding component variable of v.
If g(v) denotes a formula, I will let g(c) denote g(v)[c/v].
INTRPT == The set of mappings [LVBL -> VAL].
INTRPT is the set of interpretations--substitutions of values
for logical variables. For any set S, an element of the set of
mappings [INTRPT -> S] is an object that yields an element of S
after substituting values for all logical variables.
Expressibility and Completeness Assumptions
-------------------------------------------
Below are the expressibility assumptions and the semantic
interpretations of those assumptions. These semantic
interpretations provide a semantics for the (nontemporal) logic of
actions. In the following, I assume
s, t :in: ST
x :in: PVBL^k
c :in: VAL^k
v :in: LVBL^k
P, Q :in: PRED
A, B :in: ACT
f, g :in: SFCN
Formally, the assumptions are universally quantified over these
objects. The semantic domains are defined as follows, where [[O]]
denotes the "meaning" of an object O.
[[x]] : ST -> VAL^k
[[f]] : ST -> [INTRPT -> VAL]
[[P]] : ST -> [INTRPT -> {TRUE, FALSE}]
[[A]] : ST :X: ST -> [INTRPT -> {TRUE, FALSE}]
The following notation is used in place of the semantic brackets
[[ ]].
s.x == [[x]](s)
s.f == [[f]](s)
s.P == [[P]](s)
s.A.t == [[A]](s,t)
Thus, for example, s.P is an object that yields a Boolean after
substituting values for all logical variables. Validity of
an action formula is defined by
|= A == :A: int :in: INTRPT :
:A: s,t :in: ST : s.A.t(int)
Thus, validity of A means that s.A.t is true for all substitutions
of values for logical variables and all states s and t.
The following are the assumptions and their meanings.
EX(-1). TRUE :in: PRED
s.TRUE == TRUE
EX0. P, P' :in: ACT
P :in: SFCN
s.P.t == s.P
s.P'.t == t.P
A predicate is identified with an action that does not depend on
the second [new] state. Since VAL contains the elements TRUE
and FALSE, a predicate P is a fortiori "semantically" a state
function. The assumption P :in: SFCN states that this semantic
state function is expressible.
EX1. sin(A,P) :in: PRED
sp(A,P) :in: PRED,
s.sp(A,P) == :E: t : t.P /\ t.A.s
s.sin(A,P) == :E: i :geq: 0 : s.sp^i(A,P)
where sp^0(A,P) == P
sp^[i+1](A,P) == sp(A,sp^i(A,P))
The operators sp and sin are the usual strongest postcondition
and strongest invariant operators.
EX2. A /\ B, ~A, A \/ B :in: ACT
P /\ Q, ~P, P \/ Q :in: PRED
s.(A /\ B).t == s.A.t /\ s.B.t
s.(A \/ B).t == s.A.t \/ s.B.t
s.(~A).t == ~(s.A.t)
The corresponding semantic definitions for predicates follow
from EX2 and EX0.
EX6. x = w :in: PRED, and ~ |= ((:E: w : (x = w)) :equiv: FALSE)
s.(x = w) == s.x = w
The assumption invalidity assumption asserts that given any
finite set of variables x_i and values v_i, there is a state
in which each x_i has the value v_i.
EX7. Only a finite number of program variables appear in A.
:E: k, x : :A: r, s, t, u :in: ST :
(r.x = t.x /\ s.x = u.x) => r.A.s = t.A.u
In the semantic definition, x is any k-tuple of program
variables whose components include all the variables that appear
in A. (Since there is no quantification over program variables
in actions, any variable that appears in A is free in A.)
EX7b. Only a finite number of logical variables appear
free in A.
:E: w :in: LVBL^k :
v disjoint from w => s.A.t :equiv: s.A[c/v].t
In the semantic definition, w is a k-tuple of logical variables
containing all logical variables occurring free in A.
EX8. (a) NAT :in: SFCN
(b) If S :in: SFCN and u :in: LVBL then (u :in: S) :in: PRED
(c) If Q :in: PRED, x :in: PVBL, w :in: LVBL then
Q[w/x] :in: PRED
(d) v > w :in: CONSTPRED, then
Well-Founded(v>w, VAL^k) :in: CONSTPRED.
CONST PRED = set of all PRED's with no free logical
variables. So, Q :in: CONSTPRED iff
(:E: s : s.Q) :equiv: |= Q
If w :in: LVLB^k has components disjoint from v, and
w>v :in: PRED, then Well-Founded(w>v, VAL^k) :in: PRED.
s.Well-Founded(w>v, S) ==
~ :A: i :geq: 0 : :E: c_i :in: S : s.(c_[i+1] > c_i)
where S :subseteq: VAL^k
(c_[i+1] > c_i) == ((w > v)[c_[i+1]/w])[c_i/v]
This is the formal definition of well-founded ordering > on a
set S of k-tuples of values. Usually, w>v will be a "constant"
relation--one that is independent of the state.
EX9. If w :in: LVBL^k, then (:E: w : P) :in: PRED.
s.(:E: w : P) == :E: c :in: VAL^k : s.P[c/w]
EX10. f' = f :in: ACT
s.(f' = f).t == t.f = s.f
EX11. (f,g) :in: SFCN.
s.(f,g) == (s.f, s.g)
Note that if SFCN contains any reasonably rich set of state
functions, then this assumption requires that VAL contains all
ordered pairs of elements in VAL.
Relative Completeness Assumption
---------------------------------
A logical system consists of a set of wff's and a collection of
rules for proving that certain wff's are theorems. We usually let
|- F denote that the wff F is a theorem of the system. Since we
are considering two logical systems, the logic of actions and TLA,
there are two logical systems and two "|-"s. We'll use |-_[ACT]
for the logic of actions.
The relative completeness assumption is:
RC1. For all A :in: Act: (|= A) :equiv: |-_[ACT] A
This of course assumes the soundness as well as the completeness of
the proof system for actions.
TEMPORAL LOGIC
--------------
Notation
--------
[A]_f == A \/ (f' = f)
****_f == ~[~A]_F
= A /\ (f' # f)
ST^[:omega:] == The set of infinite sequences of elements in ST.
For :sigma: :in: ST^[:omega:] , i :geq: 0 :
:sigma:_i == The ith state in :sigma:, where the first state
is :sigma:_0.
:sigma:^[+i] == The sequence :sigma:_i, :sigma:_[i+1], ...
in ST^[:omega:]
Temporal Logic
--------------
The wffs of a simple temporal logic consist of a set of formulas
TEMPORAL defined in terms of a set ELEM of elementary formulas by
the following BNF grammar:
TEMPORAL == ELEM | ~TEMPORAL | TEMPORAL \/ TEMPORAL
| TEMPORAL /\ TEMPORAL | []TEMPORAL
In the following, I assume
F, G :in: TEMPORAL
:sigma: :in: ST^[:omega:]
The semantic domain for temporal formulas is defined by
[[F]] : ST^[:omega:] -> (INTRPRT -> {TRUE, FALSE})
I will eliminate the semantic brackets by writing
:sigma: |= F == [[F]](:sigma:)
The semantics of the temporal logic is defined in terms of the
semantics of elementary formulas by:
:sigma: |= ~F == ~(:sigma: |= F)
:sigma: |= F \/ G == (:sigma: |= F) \/ (:sigma: |= G)
:sigma: |= F /\ G == (:sigma: |= F) /\ (:sigma: |= G)
:sigma: |= []F == :A: i :geq: 0 : :sigma:^[+i] |= F
Validity is defined by
|= F == :A: :sigma: :in: ST^[:omega:] : :sigma: |= F
The temporal logic RAW is defined by letting ELEM == ACT,
where, for A :in: ACT,
:sigma: |= A == :sigma:_0.A.:sigma:_1
Since an action A in ACT is a RAW formula, we have defined |= A
twice: once in defining the semantics of actions, and just now in
defining the semantics of RAW formulas. It is easy to check that
the two definitions are equivalent.
The temporal logic TLA is defined by letting
TACT == [ACT]_[SFCN] | PRED /\ [ACT]_[SFCN]
ELEM == PRED | []TACT
It follows from EX2, and EX10 that TACT is a subset of ACT, so EX0
implies that TLA is a subset of RAW. The semantics of TLA are then
defined by the semantics of RAW.
Note: We would get the same logic by defining ELEM just to be PRED
or [ACT]_[SFCN]. However, that would leave us in the somewhat
embarrassing position of being able to write the formula
[]P /\ [][A]_f but not being able to write the equivalent formula
[](P /\ [][A]_f).
The following derived operators are defined, for any F, G :in: RAW.
<>F == ~[]~F
F ~> G == [](F => <>G)
Expressibility assumption EX2 implies <>_f :in: TLA for any
A :in: ACT and f :in: SFCN.
AXIOMS AND DEDUCTION RULES FOR TLA
----------------------------------
We let |-_[TLA] denote the provability relation for TLA.
The first deduction rule is:
STL0. For any P :in: PRED: (|-_[ACT] P) => (|-_[TLA] P)
Assuming that all our complete logical system for TLA is sound, so
|-_[TLA] P implies |= P, it follows from the relative completeness
assumption RC that |-_[TLA] P implies |-_[ACT] P. Hence, STL0
implies that |-_[TLA] and |-_[ACT] are equivalent for predicates in
PRED. Since these predicates are the only elements of both ACT and
TLA, we will drop the subscripts and use the same provability symbol
|- for both actions and TLA formulas.
In the following rules and axioms, F and G are assumed to be
arbitrary formulas in TLA.
The next part of the logical system of TLA consists of modus ponens
(|- F and |- (F => G) imply |- G) and the axioms of propositional
calculus. Instead of giving these axioms explicitly, we simply
state the following rule:
PROPCALC: If F is derivable by Modus Ponens and substitution
of TLA formulas for atoms in tautologies of
propositional logic, then |- F.
A rule of the form F, G |- H means that |- H can be derived from
|- F and |- G. We sometimes write this rule in the form
F, G
----
H
The remaining axioms and rules are:
STL1. |- [](F /\ G) :equiv: []F /\ []G
STL2. |- [][]F = []F
STL4. |- <>[]F /\ <>[]G :equiv: <>[](F /\ G)
STL6. |- []<>[]F = <>[]F
STL7. (F => G) |- ([]F => []G)
STL8. |- ([]F => F)
TLA1. For all P :in: PRED : |- ([]P :equiv: P /\ [][P => P']_[P])
TLA2. For all A, B :in: TACT :
|-(A => B) => |-([]A => []B)
LATTICE:
w :in: LVBL^k
:E: w :in: S
w not free in F
Well-Founded(<, S)
(F /\ w :in: S) => (P(w) ~> :E: v : (v :in: S) /\ (v < w) /\ P(v))
------------------------------------------------------------------
~F
Note: All these rules and axioms are valid for formulas F and G in
RAW, not just for formulas in TLA. Extending the rules and axioms
to RAW would Make TLA2 a special case of STL7.
The following are derived rules and axioms.
INV : For P :in: PRED, A :in: TACT :
(|- P /\ A => P') => (|- P /\ []A => []P)
1. (|- P /\ A => P') => (|- []A => [][P => P']_P)
Pf: |- P /\ A => P'
=> |- A => (P => P') (PROPCALC)
=> |- A => [P => P']_P (PROPCALC)
=> |- []A => [][P => P']_P (TLA2)
2. (|- P /\ A => P') => (|- P /\ []A => P /\ [][P => P']_P)
Pf: 1 and PROPCALC.
3. QED
Pf: 2, TLA1, and PROPCALC.
Lemma PRETACT: A, B :in: TACT => A /\ B :in: TACT
Pf: ...
PROG: For P, Q :in: PRED, A :in: TLACT, B :in: ~TLACT:
If |- P /\ A => P' and |- P /\ A /\ B => Q', then
|- []A /\ []<>B=> (P ~> Q)
Assume: A. |- P /\ A => P'
B. |- P /\ A /\ B => Q'
To Prove: |- []A /\ []<>B=> (P ~> Q)
Note: I will use PROPCALC without explicit mention.
1. |- ~Q /\ [~Q => ~Q']_[~Q] => ~Q'
Pf: By RC, and
~Q /\ [~Q => ~Q']_[~Q]
:equiv: ~Q /\ ((~Q => ~Q') \/ (~Q = ~Q'))
:equiv: ~Q /\ (~Q :equiv: ~Q')
:equiv: ~Q' /\ (~Q :equiv: ~Q')
=> ~Q'
2. QED
Pf: |- P /\ A /\ B => Q' (Assumption B)
=> |- ~Q' /\ P /\ A => ~B
=> |- ~Q /\ [~Q => ~Q']_[~Q] /\ P /\ A => ~B (1)
=> |- [](~Q /\ [~Q => ~Q']_[~Q] /\ P /\ A) => []~B
(TLA2 and Lemma PRETACT)
=> |- [](~Q) /\ [][~Q => ~Q']_[~Q] /\ [](P /\ A) => []~B
(STL1)
=> |- [](~Q) /\ []P /\ []A => []~B (TLA1 and STL8)
=> |- []P /\ []A /\ <>B => <>Q (def of <>)
=> |- []P /\ []A /\ []<>B => <>Q (STL8)
=> |- P /\ []A /\ []<>B => <>Q (Assumption B, INV)
=> |- []A /\ []<>B => (P => <>Q)
=> |- [](A /\ <>B) => (P => <>Q) (STL1, def of ~>)
STL3. F |- []F
Pf: From STL7, with F -> TRUE, G -> F, STL0, and PROPCALC.
NOTE ADDED 17 Nov 93: I think this proof is wrong because we
can't deduce |- []true from STL0 and PROPCALC. I think the
best fix is to change the conclusion of STL0 to []F.
STL5. |- []<>F \/ []<>G :equiv: [](<>F \/ <>G)
Pf: []<>F \/ []<>G
:equiv: ~(~[]<>F /\ ~[]<>G) (PROPCALC)
:equiv: ~(<>[]~F /\ <>[]~G) (Def of <>)
:equiv: ~(<>[](~F /\ ~G) (STL4)
:equiv: ~(<>[]~(F \/ G) (PROPCALC)
:equiv: [](<>F \/ <>G) (Def of <>)
STL9. |- []F /\ []<>G => <>(F /\ G)
PF: []F /\ []<>G
=> []<>F /\ []<>G STL14)
=> []<>(F /\ G) (STL5)
=> <>(F /\ G) (STL8)
STL10. F => G |- <>F => <>G
Pf: Assume: |- F => G
To Prove: |- <>F => <>G
1. |- ~G => ~F
Pf: PROPCALC
2. |- []~G => []~F
Pf: 1 and STL7
3. |- ~[]~F => ~[]~G
Pf: PROPCALC
4. QED
Pf: 3 and def of <>
STL11. ((P /\ F) => <>Q) |- []F => (P ~> Q)
Pf: |- P /\ F => <>Q
=> |- F => (P => <>Q) (PROPCALC)
=> |- []F => (P ~> Q) (STL7 and Def of ~>)
STL12. F => G |- F ~> G
Pf: |- F => G
=> |- [](F => G) (STL8)
=> |- [](F => <>G) (STL8)
:equiv: |- F ~> G (Def of ~>)
STL13. |- (F ~> G) /\ (G ~> H) => (F ~> H)
Pf: ...
STL14. |- F => <>F
Pf: By STL8, with F replaced by ~F.
STL15. |- <><>F :equiv: <>F
Pf: By STL2, with F replaced by ~F,
STL16. |- <>[]~TRUE :equiv: ~TRUE
|- []<>TRUE :equiv: TRUE
STL17. |- []FALSE :equiv: FALSE
STL18. (|- F => G) => |- F ~> G
The following result asserts that these rules are all sound.
Proposition SOUND: :A: F :in: TLA : (|- F) => (|= F)
Proof: Obvious (?!).
Classes of TLA Formulas
----------------------------
We now define some classes of formulas. We introduce the obvious
notation by which, for any sets :F: and :G: of formulas, :F: => :G:
denotes the set of all formulas F => G with F :in: :F: and
G :in: :G:. Thus, for example :F: \/ :F: denotes all formulas
F \/ G with F and G in :F:, which is not the same as :F:.
For any set :F: of formulas, :F:^* is defined to be the set
of finite conjunctions of formulas in :F:. More precisely,
:F:^0 == {TRUE}
:F:^[i+1] == :F:^i /\ :F:, for i :geq: 0
:F:^* == :E: i :geq: 0 : :F:^i
In TLA, a program is represented by a formula F of the form
P /\ [][N]_x /\ WF_1 /\ ... /\ WF_m /\ SF_1 /\ ... /\ SF_n
where WF_i = []<>_x \/ []<>~Enabled(_x)
SF_i = []<>_x \/ <>[]~Enabled(_x)
A_i => N
B_i => N
and x is a k-tuple of program variables. Such a formula F is
"machine closed", meaning that for any formula G that represents a
safety property, |= F => G iff |= P /\ [][N]_x => G.
All TLA properties that we prove have the form F => G for such an
F. To prove that one program implies another, we prove a formula
F => G where F is a formula representing a program, and G is
obtained from a formula representing a program by substituting
state functions for the program variables. Because of this
substitution, G need not be machine closed.
There are thus two classes of "program formulas" of interest,
machine-closed formulas of the class defined above, and the more
general class of formulas obtainable from machine-closed formulas
by substituting state functions for variables. We abstract and
generalize these two sets of formulas by the formulas PGM and
MCPGM.
We define the class PGM by
PGM == PRED /\ []TACT /\ WF^* /\ SF^*
where WF == []<>~TACT \/ []<>~TACT
SF == []<>~TACT \/ <>[]TACT
We define MCPGM to be the subset of PGM consisting of all
formulas F :in: PGM satisfying the following condition
:E: G :in: PRED /\ []TACT :
/\ (|- F => G)
/\ :A: A :in: TACT : (|= F => []A) => (|= G => []A)
The Completeness Theorem
------------------------
A logic is relatively complete for a formula set of formulas
:F: iff, for every formula F :in: :F:, if F is valid then
it is provable. We write this as Comp(:F:), defined by
Comp(:F:) == :A: F :in: :F: : (|= F) => (|- F)
The relative completeness assumption RC asserts Comp(ACT).
Our completeness result is:
THEOREM:
1. Comp(MCPGM => PRED)
2. Comp(MCPGM => []TACT)
3. Comp(MCPGM => (PRED ~> PRED))
4. Comp(MCPGM => PGM)
THE PROOF
---------
The Relation [=
---------------
We define a "provable subset" relation [= among sets of formulas,
where :F: [= :G: means that every formula in :F: is provably
equivalent to a formula in :G:.
:F: [= :G: == :A: F :in: :F: :
:E: G :in: :G: : |- (F :equiv: G)
:F: [=] :G: == (:F: [= :G:) /\ (:G: [= :F:)
Lemma PSUB: For any subsets :F:, :F:_i, and :G:_i of TLA:
1. :F: [= :F:
2. (:F:_1 [= :F:_2) /\ (:F:_2 [= :F:_3) => (:F:_1 [= :F:_3)
3. (:F:_1 [= :G:_1) /\ (:F:_2 [= :G:_2)
=> /\ ~:F:_1 [= ~:G:_1
/\ (:F:_1 /\ :F:_2) [= (:G:_1 /\ :G:_2)
/\ (:F:_1 => :F:_2) [= (:G:_1 => :G:_2)
4. (:A: i :geq: 0 : :F:_1 [= :G:_i) => F^* [= G^*
Pf: This follows easily from PROPCALC.
Lemma PSUBCOMP and ANDCOMP: For any subsets :F: and :G: of TLA:
0. (:F: [= :G:) /\ Comp(:G:) => Comp(:F:).
1. Comp(:F:) :equiv: Comp(:F:^*)
2. {TRUE} [= :G: => (Comp(:F: => :G:^*) :equiv: Comp(:F: => :G:))
Pf of 0:
(:F: [= :G:) /\ Comp(:G:)
:equiv: /\ :A: F :in: :F: : :E: G :in: :G: : |- F :equiv: G
\\\\\\ /\ :A: G :in: :G: : |= G => |- G
=> :A: F :in: :F: : :E: G :in: :G: :
(|- F :equiv: G) /\ (|= G => |- G)
=> :A: F :in: :F: : |= F =>
:E: G :in: :G: : (|- F :equiv: G) /\ (|= G => |- G)
=> :A: F :in: :F: : |= F =>
:E: G :in: :G: : (|- F :equiv: G) /\ |- G (Prop SOUND)
=> :A: F :in: :F: : |= F => |- F (PROPCALC)
:equiv: Comp(:F:)
Pf of 1:
1.1. Comp(:F:^*) => Comp(:F:)
Pf: By part 0, since :F: [= :F:^*
1.2. Comp(:F:) => Comp(:F:^*)
1.2.1. Comp({TRUE})
Pf: PROPCALC implies |- TRUE.
1.2.2. For i > 0, Comp(:F:) /\ Comp(:F:^i) => Comp(:F:^[i+1])
Assume: Comp(:F:), Comp(:F:^i), F :in: :F:, and G :in: :F:^i
To Prove: |= F /\ G => |- F /\ G
Pf: |= F /\ G
=> |= F /\ |= G (def of |=)
=> |- F /\ |- G (assumption)
=> |- F /\ G (PROPCALC)
1.2.3. QED
Pf: 1.2.1, 1.2.2, Induction, and definition of :F:^*.
1.3. QED
Pf of 2:
2.1. Comp(:F: => :G:) => Comp(:F: => :G:^*)
Pf: By Part 0, since (:F: => :G:^*) [= (:F: => :G:).
2.2. ({TRUE} [= :G:) /\ Comp(:F: => :G:^*) => Comp(:F: => :G:)
2.2.1. ({TRUE} [= :G:) =>
(:F: => :G:^0) [= (:F: => :G:)
2.2.2. (:F: => :G:^i) [= (:F: => :G:)^i
=> (:F: => :G:^[i+1]) [= (:F: => :G:)^[i+1]
Pf: By PROPCALC, which implies
|- (F => (G_1 /\ G_2)) :equiv: (F => G_1) /\ (F => G_2)
2.2.3. ({TRUE} [= :G:) => ((:F: => :G:^*) [= (:F: => :G:)^*)
Pf: 2.2.1, 2.2.2, and mathematical induction.
2.2.4. ({TRUE} [= :G:) /\ Comp((:F: => :G:)^*) => Comp(:F: => :G:^*)
Pf: 2.2.3 and Part 0.
2.2.5. QED
Pf: 2.2.4 and Part 1, with (:F: => :G:)
substituted for :F:.
Lemma TACT: TACT^* [= TACT
Proof: Use Lemma PRETACT.
1. For any A and B in ACT and any f and g in SFCN:
(a) [(A \/ (f' = f)) /\ (B \/ (g' = g)]_[(f,g)] :in: TACT
(b) |= ([A]_f /\ [B]_g
:equiv: [(A \/ (f' = f)) /\ (B \/ (g' = g)]_[(f,g)])
Pf: Part (a) follows from EX2, EX10, and EX11. Part (b) follows,
by a simple calculation, from the definition of validity
for actions.
2. QED
Pf: Follows easily by induction from 1 and EX2.
Reduction and Completeness
--------------------------
Lemma REDDEF: For any subsets :F: and :G: of TLA:
:A: F :in: :F: : :E: G :in: :G: : /\ (|= F) => (|= G)
/\ (|- G) => (|- F)
=> (Comp(:G:) => Comp(:F:))
Pf: Trivial.
Lemma COMPRED:
Comp(:F:) /\ Comp(:G:) => Comp(:F: /\ :G:)
Pf: Trivial.
Lemma BOXRED: For any sets :F: and :G: of formulas in TLA:
Comp([]:F: => :G:) => Comp([]:F: => []:G:)
Assume: A. Comp([]:F: => :G:)
B. F :in: :F:, G :in: :G:, and |= []F => []G
To Prove: |- []F => []G
1. |= []F => G
Pf: Assumption B, STL 8 and Lemma SOUND.
2. |- ([]F => G) => |- ([]F => []G)
Pf: STL7 and STL2.
3. QED
Pf: 1, 2, and assumptions.
Lemma SIN: For any P :in: PRED, A :in: TACT, G :in: TLA,
and F :in: WF^* /\ SF^*:
1./\ |= (P /\ []A /\ F => []G) => |= [](sin(A, P) /\ []A /\ F => []G)
2./\ |- [](sin(A, P) /\ []A /\ F => []G) => |- (P /\ []A /\ F => []G)
Pf: LET I == sin(A, P)
Assume P :in: PRED, A :in: TACT, ...
0. For all :tau: :in: ST^[:omega:] and n :geq: 0 :
(:tau: |= F) :equiv: (:tau:^[+n] |= F)
Pf: Assume: :tau: :in: ST^[:omega:] , n :geq: 0 ,
0.1. :A: H :in: []<>~TACT \/ <>[]TACT :
(:tau: |= H) :equiv: (:tau:^[+n] |= H)
Pf: Follows easily from definitions.
0.2. :A: H :in: []<>~TACT \/ []<>TACT :
(:tau: |= H) :equiv: (:tau:^[+n] |= H)
Pf: Follows easily from definitions.
0.3. QED
Pf: By 0.1 and 0.2, since :sigma: |= H_1 /\ H_2 equals
:sigma: |= H_1 /\ :sigma: |= H_2 by def of |=.
1. |- P /\ []A => []I
1.1. |- P => I
Pf: |= P => I and Relative Completeness Assumption 2.
1.2. |- I /\ A => I'
1.2.1. ...
1.3. QED
Rule INV
2. :A: :sigma: :in: ST^[:omega:] :
:sigma: |= []I /\ []A /\ F
=> :E: :tau:, n : /\ :sigma: = :tau:^[+n]
\\\\\\ /\ :tau: |= P /\ []A /\ F
Assume: 3A. :sigma: |= []I /\ []A /\ F
To Prove: :E: :tau:, n : /\ :sigma: = :tau:^[+n]
/\ :tau: |= P /\ []A /\ F
2.1. :sigma:_0.I
Pf: by assumption, definition of |= and [].
2.2. Choose :tau:_0, ..., :tau:_[n] such that
:tau:_0.P, :tau:_[i-1].A.:tau:_i,
and :tau:_[n] = :sigma:_0.
Pf: 2.1, definition of sin(P, A).
2.3. Let :tau:_[n+i] == :sigma:_i, then
:tau: |= P /\ []A
Pf: By 2.2 and assumption 3A.
2.4. :tau: |= F
Pf: :sigma: |= F by assumption 3A, and
:tau: |= F follows by 0.
2.5. QED
Pf: 2.3, 2.4.
3. |= (P /\ []A /\ F => []G)
=> |= ([]I /\ []A /\ F => []G)
3.1. :A: :sigma: :
/\ |= (P /\ []A /\ F => []G)
/\ :sigma: |= ([]I /\ []A /\ F)
=> :sigma: |= []G
Assume: 4A.1. |= (P /\ []A /\ F => []G)
4A.2. :sigma: |= ([]I /\ []A /\ F)
To Prove: :sigma: |= []G
3.1.1. Choose :tau: s.t.
1./\ :sigma: = :tau:^[+n]
2./\ :tau: |= P /\ []A /\ F
Pf: By 2.
3.1.2. :tau: |= []G
Pf: By assumption 4A.1, 3.1.1.2, and definition
of |=.
3.1.3. QED
By 3.1.2, 3.1.1.1, and definition of |= []G.
3.2. QED
By 3.1, since
|= ([]I /\ []A /\ F => []G) ==
:A: :sigma: : :sigma: |= ([]I /\ []A /\ F)
=> :sigma: |= []G
4. |- ([]I /\ []A /\ F => []G) => |- (P /\ []A /\ F => []G)
Pf: 1 and PROPCALC.
5. QED
3 and 4.
Lemma SINRED: For any set :F: of formulas in TLA:
Comp([]TACT /\ WF* /\ SF* => []:F:) => Comp(PGM => []:F:)
Pf: Lemma SIN and Lemma REDDEF.
Lemma R1. /\ Comp(MCPGM => PRED)
/\ Comp(MCPGM => []TACT)
/\ Comp(MCPGM => WF)
/\ Comp(MCPGM => SF)
=> Comp(MCPGM => PGM)
Pf: 1. /\ Comp(MCPGM => PRED)
/\ Comp(MCPGM => []TACT)
/\ Comp(MCPGM => WF^*)
/\ Comp(MCPGM => SF^*)
=> Comp(MCPGM => PGM)
Pf: Lemma COMPRED and definition of PGM.
2. /\ Comp(MCPGM => WF) => Comp(MCPGM => WF^*)
/\ Comp(MCPGM => SF) => Comp(MCPGM => SF^*)
Pf: By Lemma ANDCOMP, since STL16 implies
{TRUE} [= WF and {TRUE} [= SF.
3. QED
Pf: 1 and 2.
Lemma WFRED: WF [= SF
1. F :in: WF :equiv: :E: A, B :in: TACT : F = []<>~A \/ []<>~B
Pf: Def of WF.
2. :A: A, B :in: TACT :
|- ([]<>~A \/ []<>~B) :equiv: ([]<>~(A /\ B) \/ <>[]~TRUE)
Pf: STL5,, STL16, and PROPCALC.
3. F :in: WF => :E: G :in: SF : |- F :equiv: G
Pf: 1, 2, EX2, EX-1, and def of SF.
4. QED
Pf: 3 and def of [=
Lemma IORED: For any sets :F:, :G: and :H: of formulas in TLA:
Comp([]:F:^* /\ []:G: => []:H:) =>
Comp([]:F:^* /\ (<>[]:F:)^* /\ []:G: => []:H:)
1. []:F:^* /\ (<>[]:F:)^* /\ []:G: => []:H:
[= [](:F:^*) /\ []:G: => [](<>~(:F:^*) /\ []:H:) (STL1 and STL6)
Pf: []:F:^* /\ (<>[]:F:)^* /\ []:G: => []:H:
[= [](:F:^*) /\ <>[](:F:^*) /\ []:G: => []:H: (STL4)
[=] [](:F:^*) /\ []:G: => ([]<>~(:F:^*) /\ []:H:) (PROPCALC)
2. [](:F:^*) /\ []:G: => <>~(:F:^*) /\ []:H:
[= [](:F:^*) /\ []:G: => []:H:
Pf: [](:F:^*) /\ []:G: => <>~(:F:^*) /\ []:H:
[=] [](:F:^*) /\ [](:F:^*) /\ []:G: => []:H: (PROPCALC)
[=] [](:F:^*) /\ []:G: => []:H: (STL1)
3. QED
Pf: Comp([]:F:^* /\ []:G: => []:H:)
=> Comp([]:F:^* /\ (<>[]:F:)^* /\ []:G: => []:H:)
(2 and PSUBCOMP.0)
=> Comp([](:F:^*) /\ []:G: => <>~(:F:^*) /\ []:H:)
(1 and PSUBCOMP.0)
=> Comp([](:F:^*) /\ []:G: => [](<>~(:F:^*) /\ []:H:))
(STL1 and Lemma BOXRED)
=> Comp([]:F:^* /\ (<>[]:F:)^* /\ []:G: => []:H:)
1 and Lemma PSUBCOMP.0.
Lemma PGMRED: For any set :F: of formulas in TLA:
Comp([]TACT /\ ([]<>~TACT)^* => []:F:) => Comp(PGM => []:F:)
1. For any subsets :F:, :G:, :H:, and :I: of TLA:
:F: /\ (:G: \/ :H:)^* => :I:
[= (:F: /\ :G:^* /\ :H:^* => :I:)^*
1.1. :F: /\ (:G: \/ :H:)^0 => :I:
[= (:F: /\ :G:^* /\ :H:^*)^* => :I:)^*
Pf: Trivial
1.2. If :F: /\ (:G: \/ :H:)^i => :I:
[= (:F: /\ :G:^* /\ :H:^* => :I:)^*
then :F: /\ (:G: \/ :H:)^[i+1] => :I:
[= (:F: /\ :G:^* /\ :H:^* => :I:)^*
Pf: :F: /\ (:G: \/ :H:)^[i+1] => :I:
[=] :F: /\ (:G: \/ :H:)^i /\ (:G: \/ :H:) => :I:
[=] /\ :G: => (:F: /\ (:G: \/ :H:)^i => :I:)
/\ :H: => (:F: /\ (:G: \/ :H:)^i => :I:)
[= /\ :G: => (:F: /\ :G:^* /\ :H:^* => :I:)^*
/\ :H: => (:F: /\ :G:^* /\ :H:^* => :I:)^*
[= /\ (:G: => (:F: /\ :G:^* /\ :H:^* => :I:))^*
/\ (:H: => (:F: /\ :G:^* /\ :H:^* => :I:))^*
[= /\ (:F: /\ :G:^* /\ :H:^* => :I:)^*
/\ (:F: /\ :G:^* /\ :H:^* => :I:)^*
[=] (:F: /\ :G:^* /\ :H:^* => :I:)^*
1.3. QED
Pf: 1.1, 1.2, and mathematical induction.
2. []TACT /\ WF^* /\ SF^* => []:F:
[= ([]TACT /\ ([]<>~TACT)^* => []:F:)
Pf: []TACT /\ WF^* /\ SF^* => []:F:
[= ([]TACT /\ ([]<>~TACT)^* /\ SF^* => []:F:)
(By 1, with :G: = :H: = []<>~TACT)
[= ([]TACT /\ ([]<>~TACT)^* /\ (<>[]TACT)^* => []:F:)
(By 1, with :G: = []<>~TACT, :H: = ...)
[= ([]TACT /\ ([]<>~TACT)^* => []:F:)
(Lemma IORED and STL1)
3. Comp([]TACT /\ ([]<>~TACT)^* => []:F:)
=> Comp([]TACT /\ WF^* /\ SF^* => []:F:)
PF: 2 and Lemma PSUBCOMP.0.
4. Comp([]TACT /\ WF^* /\ SF^* => []:F:) => (PGM => []:F:)
Pf: Lemma SINRED.
5. QED
Pf: 3 and 4.
Lemma R2. Comp(PGM => SF) => Comp(PGM => WF)
1. (PGM => WF) [= (PGM => SF)
Pf: Lemma WFRED and Lemma PSUB.3.
2. QED
Pf: 1 and Lemma PSUBCOMP.0.
Lemma R3. Comp(~([]TACT /\ ([]<>~TACT)^*)) => Comp(PGM => SF)
1. Comp(~([]TACT /\ ([]<>~TACT)^*))
=> Comp([]TACT /\ ([]<>~TACT)^* => <>[]TACT)
Pf: By Lemma PSUBCOMP.0, since
[]TACT /\ ([]<>~TACT)^* => <>[]TACT
[=] ~([]TACT /\ ([]<>~TACT)^* /\ []<>~TACT)
[= ~([]TACT /\ ([]<>~TACT)^*)
2. Comp([]TACT /\ ([]<>~TACT)^* => <>[]TACT)
=> Comp([]TACT /\ ([]<>~TACT)^* /\ <>[]TACT => <>[]TACT)
Pf: Lemma IORED and STL6.
3. Comp([]TACT /\ ([]<>~TACT)^* /\ <>[]TACT => <>[]TACT)
=> Comp([]TACT /\ ([]<>~TACT)^* => SF)
Pf: By Lemma PSUBCOMP.0, since
[]TACT /\ ([]<>~TACT)^* => SF
[=] []TACT /\ ([]<>~TACT)^* => []<>~TACT \/ <>[]TACT
[=] []TACT /\ ([]<>~TACT)^* /\ <>[]TACT => <>[]TACT
4. Comp([]TACT /\ ([]<>~TACT)^* => SF) => Comp(PGM => SF)
Pf: By Lemma PGMRED, since STL5 and STL2 imply SF [=] []SF.
5. QED
Pf: 1 - 4 and transitivity of =>.
Lemma R4. Comp(~([]TACT /\ ([]<>~TACT)^*)) =>
/\ Comp(PGM => PRED)
/\ Comp(PGM => (PRED ~> PRED))
1. Comp(~([]TACT /\ ([]<>~TACT)^*)) => Comp(PGM => FALSE)
Pf: By Lemma PGMRED and STL17.
2. Comp(PGM => FALSE) => Comp(PGM => PRED)
Pf: By Lemma PSUBCOMP.0, since
PGM => PRED
[=] ~PRED /\ PGM => FALSE
[= PGM => FALSE
3. Comp(PGM => PRED) => Comp(PGM => (PRED => <>PRED)
Pf: By Lemma PSUBCOMP.0, since
PGM => (PRED => <>PRED)
[=] (PGM /\ ~<>PRED) => ~PRED
[=] (PGM /\ []PRED) => PRED
[= PGM => PRED (By Lemma TACT)
4. Comp(PGM => (PRED => <>PRED) => Comp(PGM => (PRED ~> PRED))
Pf: By Lemma SINRED, Lemma PSUBCOMP.0, and Lemma PSUB.3, since
[]TACT /\ WF^* /\ SF^* [= PGM.
5. QED
Pf: 1 - 4.
Lemma R5. Comp(MCPGM => []TACT)
1. Comp(PRED /\ []TACT => []TACT) => Comp(MCPGM => []TACT)
Pf: By definition of MCPGM.
2. Comp([]TACT => []TACT) => Comp(PRED /\ []TACT => []TACT)
Pf: By Lemma SIN and Lemma REDDEF.
3. Comp([]TACT => []TACT)
Assume: F :in: []TACT => []TACT, |= F
To Prove: |- F
3.1. For all G :in: []TACT there exist P :in: PRED,
A :in: ACT, and f :in: SFCN such that
|- G :equiv: [](P /\ [(P = P') /\ A]_(f,P)).
Pf: It follows from TLA1, STL8, and the definition of [A]_f
that
|- [](P /\ [][B]_f) :equiv:
[](P /\ [(P'=P) /\ (B \/ (f'=f)]_(f,P))
The result then follows from the definition of
TACT, EX(-1), and |= [TRUE]_f :equiv: TRUE,
which by Assumption RC implies |- [TRUE]_f :equiv: TRUE.
3.2. Choose P, Q :in: PRED; A, B :in: ACT, and
f, g :in: SFCN such that
F = [](P /\ [(P'=P) /\ A]_f) => []B
Pf: By 3.1.
3.3. |= (P /\ [(P'=P) /\ A]_f) => B
Assume: s,t :in: ST and s.(P /\ [(P'=P) /\ A]_(f,P)).t
To Prove: s.B.t
3.3.1. t.P
Pf: s.(P /\ [(P'=P) /\ A]_(f,P)).t implies
s.P and s.P = t.P.
3.3.2. t.(P /\ [(P'=P) /\ A]_(f,P)).t
Pf: By 3.3.1.
3.3.3. Define :sigma: :in: ST^[:omega:] by
:sigma:_0 = s and :sigma:_i = t for all i > 0.
Then :sigma: |= [](P /\ [(P'=P) /\ A]_f)
Pf: 3.3.2, assumption, and def of |=.
3.3.4. :sigma: |= []B
Pf: 3.3.3 and assumption |= F.
3.3.5. :sigma:_0.B.:sigma:_1
Pf: 3.3.4 and definition of |= []B.
3.3.6. QED
Pf: 3.3.5 and 3.3.3.
3.4. |- (P /\ [(P'=P) /\ A]_f) => B
Pf: 3.3 and assumption RC.
3.5. QED
Pf: 3.4 and TLA2.
4. QED
Pf: 1 - 3.
Lemma SPSIN.
1. |= sin(A, P) /\ A => sin(A, P)'
2. |= P /\ A => sp(A; P)'
Lemma MAIN: If N, ~A_1, ..., ~A_n :in: TACT, then
|= ~([]N /\ []<>A_1 /\ ... /\ []<>A_n)
=> |- ~([]N /\ []<>A_1 /\ ... /\ []<>A_n)
Assume: |= ~([]N /\ []<>A_1 /\ ... /\ []<>A_n)
To Prove: |- ~([]N /\ []<>A_1 /\ ... /\ []<>A_n)
1. Let x :in: PVBL^k include all program variables free in
N and the A_i, and let w :in: LVBL^k not include an
logical variables free in N or any of the A_i.
Pf: The existence of x and w follow from EX7 and EX7b.
2. Define
P_0(w) == x = w
P_i(w) == sin(N, sp(N /\ A_i, P_[i-1]) for 1 :leq: i :leq: n
Then P_i :in: PRED for 0 :leq: i :leq: n
Pf: EX6 and EX1.
3. Choose v :in: LVBL^k, disjoint from w and not containing
any logical variables free in N or the A_i.
Pf: EX7b.
4. Define w>v == P_n(w)[v/x]
Then |= Well-Founded(>, VAL^k).
Pf: Define a sequence s_0, ... , s_p to be G-LIVE iff
/\ :A: i :in: (0 .. p] : s_[i-1].N.s_i
/\ :A: j :in: [1 .. n] :
:E: i :in: (0 .. p] : s_[i-1].A_j.s_i
4.0. w>v :equiv: |= (x = v) => P_n(w)
4.0.1. For any Q : if u :in: LVBL^k, and u does not appear free
in Q, then Q :equiv: :A: u : (v = u) => Q[u/v]
Pf: Ordinary logic.
4.0.2. For any predicate Q,
Q[u/x] :equiv: :A: s : ((s.x = u) => s.Q)
Pf: Obvious.
4.0.3. QED
Pf: P_n(w)[v/x]
:equiv: :A: u : (u = v) /\ P_n(w)[v/x][u/v] (by 1)
:equiv: :A: u : (u = v) /\ P_n(w)[u/x] (obvious)
:equiv: :A: s : (s.x = u) => s.((x = v) /\ P_n(w)) (2)
:equiv: :A: s : s.((x = v) /\ P_n(w))
(x contains all free program variable in P_n(w))
:equiv: |=(x = v) /\ P_n(w) (def of |=)
4.1. For any state s and any d :in: VAL^k, if s.P_n(d)
then there exists a G-LIVE sequence s_0, ... , s_p
s.t. s_p = s and s_0.x = d
4.1.1. :A: j :in: [1 .. n] : t_0.P_[j](d) =>
:E: t_1, ... , t_q :
/\ t_q.sp(N /\ A_j; P_[j-1])(d)
/\ :A: i :in: (0 .. q] : t_i.N.t_[i-1]
Pf: Def of sin, since sin(...)(d) = sin(...(d)).
4.1.2. :A: j :in: [1 .. n] : t_0.sp(N /\ A; P_[j-1])(d)
=> :E: t_1 : /\ t_1.(N /\ A_j).t_0
/\ t_1.P_[j-1](d)
Pf: Def of sp.
4.1.3. t_0.P_0(d) => :E: t_1, ... , t_q :
/\ t_q.x = d
/\ :A: i :in: (0 .. q] : t_i.N.t_[i-1]
Pf: Def of sin, since sin(...)(d) = sin(...(d)).
4.1.4. QED
Pf: Use 4.1.1 - 4.1.3 to construct the sequence
backwards, starting from s.
4.2. If d>c then for any state s_0 s.t. s_0.x = d
there exists a G-LIVE sequence s_0, ... , s_p s.t. s_p.x = c.
Pf: Assume d>c and s_0.x = d.
4.2.1. If u_0, ... , u_p is a G-LIVE sequence s.t.
u_0.x = d and u_p.x = c, and t_i.x = u_i.x for
all i, then t_0, ... , t_p is a G-LIVE sequence s.t.
t_0.x = d and t_p.x = c.
Pf: Follows from hypothesis that x includes all
the free variables: more precisely, that the
x are chosen according to EX7.
4.2.2. Choose a G-LIVE sequence t_0, ..., t_p s.t.
t_0.x = d and t_p.x = c
Pf: By 4.0, d>c implies (s.x = c) => s.P_n(d).
The assumption that VAL is nonempty (it contains
TRUE) implies that there exists a state s with
s.x = c. (The nonemptiness of VAL is implied by
the existence of c if k > 0.)
4.2.3. QED
Pf: Let s_i = t_i for i > 0. Then s_0, ... , s_p is
a G-LIVE sequence by 4.2.1 and 4.2.2., and s_p.x = c.
by 4.2.2.
4.3. If c_1 > c_2 > ... then there exists :sigma: :in: ST^[:omega:]
such that
(a) :A: i :geq: 0 : :sigma:_i.N.:sigma:_[i+1]
(b) :A: j :in: [1 .. n] there exist infinitely many
j :geq: 0 such that :sigma:_i.A_j.:sigma:_[i+1]
Pf: By EX6, there exists a state s_0 such that
s_0.x = c_1. We can then apply 4.2 inductively
yp 4.2, there exist G-LIVE sequences :tau:(i) such
that the last state of :tau:(i) is the first state
of :tau:(i+1). Let :sigma: be the behavior obtained
by concatenating the behaviors :tau:(i).
4.4. QED
Pf: Assume c_1 > c_2 > ... , and let :sigma: be the sequence
obtained in 4.3. By definition of |=,
:sigma: |= []N /\ []<>A_1 /\ ... /\ []<>A_n,
contradicting the hypothesis
|= ~([]N /\ []<>A_1 /\ ... /\ []<>A_n).
5. QED
5.1. :A: j :in: [1 .. n] :
|- ([]N /\ []<>A_1 /\ ... /\ []<>A_n) => (P_[j-1] ~> P_j)
Pf: Assume j :in: [1 .. n]
LET R == sp(N /\ A; P_[j-1])
5.1.1. |= P_[j-1] /\ N => (P_[j-1])'
Pf: Lemma SPSIN.1
5.1.2. |= P_[j-1] /\ N /\ A_j => R'
Pf: Lemma SPSIN.2
5.1.3. |- []N /\ []<>A_j => (P_[j-1] ~> R)
Pf: Assumption RC and Rule PROG.
5.1.4. R => P_j
Pf: Lemma SPSIN.3
5.1.5. []N /\ []<>A_j => (P_[j-1] ~> P_j)
Pf: STL12 and STL13.
5.1.6. QED
Pf: 5.1.5 and PROPCALC
5.2. |- ([]N /\ []<>A_1 /\ ... /\ []<>A_n) => ((x = w)~> P_n(w))
Pf: 5.1 and STL13, since P_0 equals x = w by definition.
5.3. |- P_n(w) => :E: v :in: VAL^k : (w > v) /\ x = v
5.3.1. |= P_n(w) :equiv: :E: v :in: VAL^k : x = v /\ P_n(w)
Pf: Def of |=.
5.3.2. |= x = v /\ P_n(w) :equiv: (w > v)
Pf: Definition of >
5.3.3. |= P_n(w) => :E: v :in: VAL^k : (w > v) /\ x = v
Pf: 5.3.1 and 5.3.2.
5.3.4. QED
Pf: EX9 and assumption RC.
5.4. QED
Pf: 4, 5.2, 5.3, STL18, EX8, and the LATTICE Rule, with
x = w substituted for P(w), and VAL^k substituted for S.
Lemma R6. Comp(~([]TACT /\ ([]<>~TACT)^*))
Pf: Lemma MAIN and Lemma REDDEF.
PROOF OF THEOREM:
Follows from Lemmas R1 - R6, since MCPGM :subseteq: PGM.
\end{spec}
\end{document}
**