Filters are the third @lock on groups. They exist to restrict
what can be added (and removed) to (and from) the group. Note:
the FIRST lock regulates WHO can add/remove object from the group,
the second regulates the visibility of the group (if !dark).
@lock +X = me ; !#0 ; (membership restrictions)
@lock +X = me ; !#0 ; (!+X & (requirements for group membership) )
@lock +X = me ; !#0 ; ( +X & (requirements for release from membership) )
@lock +X = me ; !#0 ; (!+X & (entry reqs) ) | (+X & (release reqs) )
@LOCK +Smart = ME ; !#0 ; !+Dumb
means only me, or the things I own, can add/remove people from +Smart,
and ONLY if the person being added/removed is NOT a member of +Dumb.
@lock +Wedged = ME ; !#0 ; (!+Wedged & +falling) | (+wedged & +starved)
A person can only be added to +WEDGED (ie, with an @effect) if they are
not in +WEDGED and are in +FALLING, and can escape +WEDGED if they're in
+WEDGED and in +STARVED.