ble with weak
     keys, or a value in a hash table with weak value does not prevent
     an object from being returned by a guardian.  But as long as an
     object can be returned from a guardian it will not be removed from
     such a weak vector or hash table.  In other words, a weak link does
     not prevent an object from being considered collectable, but being
     inside a guardian prevents a weak link from being broken.

     A key in a weak key hash table can be though of as having a strong
     reference to its associated value as long as the key is accessible.
     Consequently, when the key only accessible from within a guardian,
     the reference from the key to the value is also considered to be
     coming from within a guardian.  Thus, if there is no other
     reference to the value, it is eligible to be returned from a
     guardian.

