+ * Note: when want to apply recursively, always apply the continuator
+ * on the toplevel expression, otherwise may miss some intermediate steps.
+ * Do
+ * match expr with
+ * | FunCall (e, es) -> ...
+ * k expr
+ * Or
+ * match expr with
+ * | FunCall (e, es) -> ...
+ * Visitor_c.vk_expr bigf e
+ * Not
+ * match expr with
+ * | FunCall (e, es) -> ...
+ * k e
+ *
+ *