+/* Deletes variable V from dictionary D and frees V.
+
+ This is a very bad idea if there might be any pointers to V
+ from outside D. In general, no variable in default_dict
+ should be deleted when any transformations are active, because
+ those transformations might reference the deleted variable.
+ The safest time to delete a variable is just after a procedure
+ has been executed, as done by MODIFY VARS.
+
+ Pointers to V within D are not a problem, because
+ dict_delete_var() knows to remove V from split variables,
+ weights, filters, etc. */