A flexible variable is one that can have a different
value in different states of a
behavior.
There are infinitely many flexible variables, but only a finite number
of them can occur in any single formula.
A rigid variable is one that has the same value in every
state of a
behavior. Rigid variables are the
variables of ordinary predicate logic. (As in ordinary predicate
logic, there are infinitely many such variables.) An expression
formed only from rigid variables is sometimes called a
constant.