Reimplement integer variable detection

This is a reimplementation of the integer variable detection procedure.
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.

examples/permutations.lp

@@ -1,3 +1,6 @@
+#show p/2.
+#external integer(n(0)).
+
 {p(1..n, 1..n)}.
 
 :- p(X, Y1), p(X, Y2), Y1 != Y2.
@@ -8,5 +11,3 @@ q2(Y) :- p(_, Y).
 
 :- not q1(X), X = 1..n.
 :- not q2(Y), Y = 1..n.
-
-#show p/2.