Prevent infinite loop unifying variable with itself

src/Propa/Prolog/Unify.hs

@@ -95,6 +95,7 @@ unifyTerm t1 t2 = do
   case (t1', t2') of
     (TStat s1, TStat s2)      -> unifyStat s1 s2
     (TInt i1, TInt i2)        -> guard $ i1 == i2
+    (TVar v, TVar w) | v == w -> pure ()
     (TVar v, t)               -> bindTerm v t
     (t, TVar v)               -> bindTerm v t
     (_, _)                    -> empty