call_stack - returns information about the functions involved in calling
this function

string *call_stack(int option);

If the int `option` argument is 0, call_stack() returns an array of the
names of the on the call stack, with the first one being the most recent
(i.e. the currently running program).  If the int argument is 1, call_stack
returns the objects in which that program is executing.  If it is 2, the
name of the functions are returned.  If it is 3, the value of origin() in
that frame is returned.

previous_object(3), origin(3)