[PostScript]
JoinOp (\join): trait
  % Container combining operator
  % e.g., union, concatenation
  assumes InsertGenerated
  introduces __\join __: C, C -> C
  asserts forall e: E, c, c1, c2: C
    empty \join c == c;
    insert(e, c1) \join c2 == insert(e, c1 \join c2)
  implies 
    Associative (\join, C)
    converts \join
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