This adds support for detecting integer variables in formulas.
The idea is to iteratively assume variables to be noninteger and to
prove that this would lead to a false or erroneous result. If the proof
is successful, the variable is integer as a consequence.
The implementation consists of two parts. The first one is a visitor
class that recursively searches for all declared variables in a formula
and applies the second part, a custom check. Three such checks are
implemented.
The first one tests whether a predicate definition is falsified by
making a variable noninteger, in which case it can be concluded that the
variable in question is integer. The second one checks whether bound
variables in a quantified formula turn the quantified part false, again
to conclude that variables are integer. The third check consists in
testing if making a variable noninteger turns the entire formula
obtained from completion true. In this case, the statement can be
dropped and the variable is concluded to be integer as well.
