patch 7.4.2120
Problem:    User defined functions can't be a closure.
Solution:   Add the "closure" argument. Allow using :unlet on a bound
            variable. (Yasuhiro Matsumoto, Ken Takata)
diff --git a/src/proto/userfunc.pro b/src/proto/userfunc.pro
index e503bcd..42c5883 100644
--- a/src/proto/userfunc.pro
+++ b/src/proto/userfunc.pro
@@ -46,6 +46,7 @@
 void restore_current_funccal(void *f);
 void list_func_vars(int *first);
 dict_T *get_current_funccal_dict(hashtab_T *ht);
+hashitem_T *find_hi_in_scoped_ht(char_u *name, char_u **varname, hashtab_T **pht);
 dictitem_T *find_var_in_scoped_ht(char_u *name, char_u **varname, int no_autoload);
 int set_ref_in_previous_funccal(int copyID);
 int set_ref_in_call_stack(int copyID);