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)
: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.