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Permutation (E, C): trait
  % Test for having the same elements
  assumes Container
  includes
    Bag (B for C),
    CoerceContainer (C for DC, B for RC) 
  introduces isPermutation: C, C -> Bool
  asserts forall c1, c2: C
      isPermutation(c1, c2) == coerce(c1) = coerce(c2)
  implies forall e: E, c1, c2: C
    isPermutation(c1, c2)
      => count(e, c1) = count(e, c2)
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