custom-print-functions is a variable defined in `cl-lib.el'.
Its value is

This is a list of functions that format user objects for printing.
Each function is called in turn with three arguments: the object, the
stream, and the print level (currently ignored). If it is able to
print the object it returns true; otherwise it returns nil and the
printer proceeds to the next function on the list.

This variable is not used at present, but it is defined in hopes that
a future Emacs interpreter will be able to use it.