Membership is checked in locks with the : operator, and
set by applying @EFFECTS to exits.

When checking for lack of membership the syntax should be:

 !x:y   implies something, not X, is a member of Y,
 x:!y   implies X is a member of something that isn't Y.
   both are meaningless things to test for.
 
!(x:y)  is the correct way to check if X is not a member of Y.

see also: locks, logic, operators, @effects