Member

  Member maps paths whose first element is also to be found later in the path to 1, and all other paths to 0.

    Member:
       <> == Fail:<'Member'>
       <$x> == 0
       <$x $y> == First:<Or:<First:<Equal> Member:<$x>>>.
    
    Member:<a b c> = 0.
    Member:<a b c a b c> = 1.

Definition due to Bill Keller - should work but needs to be tested.