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Member maps paths whose first element is also to be found later in the
path to 1, and all other paths to 0.
<> == 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.
Copyright © Roger Evans, Gerald Gazdar & Bill Keller, Tuesday 10 November 1998