--- flatten
#    Returns a new set that is a copy of the set, flattening each
#    containing set recursively.
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    :

      p s0 = Set[Set[1,2], 3]  #=> #<Set: {#<Set: {1, 2}>, 3}>
      p s0.flatten             #=> #<Set: {1, 2, 3}>
      p s1 = Set[Set[1,2], 1]  #=> #<Set: {#<Set: {1, 2}>, 1}>
      p s1.flatten             #=> #<Set: {1, 2}>

