here's the docstring for list-callers:
-------------------------------------------------- List the callers of FUNCTION-NAME. This function is like WHO-CALLS except that it is expected to use lower-level means. Whereas WHO-CALLS is usually implemented with special compiler support, LIST-CALLERS is usually implemented by groveling for constants in function objects throughout the heap.
The return value is as for WHO-CALLS. --------------------------------------------------
anybody remember why we make this distinction?