#pair__brace  a set of two similar things considered as a unit
  supertype:  set.aggregation  a group of things of the same kind that belong together and are so used: "a set of books"; "a set of golf clubs"; "a set of teeth"
  subtype:  Tweedledum_and_Tweedledee__Tweedledee_and_Tweedledum  any two people who are hard to tell apart

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