#sum__join  a set containing all and only the members of two or more given sets; "let C be the union of the sets A and B"
  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:  #direct_sum  a union of two disjoint sets in which every element is the sum of an element from each of the disjoint sets

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