Description: Define the membership relation (also called "epsilon relation" since it
is sometimes denoted by the lowercase Greek letter "epsilon"). Similar
to Definition 6.22 of TakeutiZaring p. 30. The membership relation
and the membership predicate agree, that is, ( AE B <-> A e. B ) ,
when B is a set (see epelg ). Thus, |- 5 E { 1 , 5 }
( ex-eprel ). (Contributed by NM, 13-Aug-1995)