SYNOPSIS
  int clean_up(int ref)

IMPLEMENTED
  Where necessary.

DESCRIPTION
  If an object/clone/super class wasn't touched for a long time, this function
  is called to give an opportunity for selfdestruction (conserving memory).

  The given <ref>count is:

    ref = -1: the object's environment would like to clean_up().
    ref =  0: the object is a clone, or a blueprint with replaced program.
    ref =  1: the object is a swapped or unused blueprint.
    ref >  1: the object is a used blueprint with <ref> references.

  Result:
    != 0: try to clean_up() again later.
    == 0: never call clean_up() again, even if the object continues to exist.

  Called by kernel objects.

