[PostScript]
MemberOp: trait
  assumes InsertGenerated
  introduces
    __ \in __, __ \notin __: E, C -> Bool
  asserts forall e, e1, e2: E, c: C
    e \notin c == ~(e \in c);
    e \notin empty;
    e1 \in insert(e2, c) == e1 = e2 \/ e1 \in c
  implies converts \in, \notin
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