Closer to Weak objects.
    Implements the specification: http://clisp.cons.org/impnotes/weak.html
    for Common Lisp implementations that have weak-pointers.
    WEAK-OR-RELATION is a primitive that cannot be implemented propertly
    without implementation support.
    Currently work on:
    clisp      full support
    cmucl      partial support (missing WEAK-OR-RELATION)
    sbcl       partial support (missing WEAK-OR-RELATION)
    Read-time Features:
    :WEAK-TEST    When testing, call the garbage collector
    to break weak pointers sooner.
    :DEBUG-WEAK   When compiling on clisp, use this implementation
    instead of the native one.