From the NannyMUD documentation

LAST CHANGE

2000-12-11

FUNCTION

NAME
        previous_object - give object that called current object


SYNTAX
	object previous_object()

DESCRIPTION
	This function returns the previous object that called a function
	in this object from the outside, ie with `call_other()'. 

	This include `call_other(this_object(), ...)' constructions. In 
	this case, the returned value will be `this_object()'.


NOTE
	If the function was called internally, the result is undefined.