fix(library/fo_unify): bug at function that extracts the lhs and rhs …

…of homogeneous/heterogeneous equality

For homogeneous equality, arg #1 is the Type of the lhs/rhs.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

src/library/fo_unify.cpp

@@ -29,7 +29,7 @@ static expr_pair eq_heq_args(expr const & e) {
     if (is_eq(e))
         return expr_pair(eq_lhs(e), eq_rhs(e));
     else
-        return expr_pair(arg(e, 1), arg(e, 2));
+        return expr_pair(arg(e, 2), arg(e, 3));
 }
 
 optional<substitution> fo_unify(expr e1, expr e2) {