A <EM>state predicate</EM> is a Boolean expression that may
contain 
  <A HREF = "variables.html">rigid and flexible variables</A>. 
Semantically, it is satisfied by a
  <A HREF = "states.html">state</A>
iff it is true when each variable is replaced with the value it is
assigned by the
  <A HREF = "states.html">state</A>.