use a test inspired from @gebner

Author: tahina-pro

test/pure_fn.c

@@ -49,3 +49,10 @@ void test() {
     bool pure_not_result = pure_not(true);
     _assert(pure_not_result == false);
 }
+
+_pure int pure_ghost_stmt()
+  _ensures(1 == 0)
+{
+  _ghost_stmt(assume False);
+  return 1;
+}