#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"
  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
  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"
     supertype:  aggregation__collection__accumulation__assemblage  several things grouped together
        supertype:  group__grouping  any number of entities (members) considered as a unit
           supertype:  collection  something gathering separated things (entities/situations)
              supertype:  divisible_entity__divisibleentity  many classifications under this category are application-dependant
                 supertype:  entity  something that can be "involved" in a situation
                    supertype:  thing__something___T__t___3D_or_4D_thing_or_anything_else  any category (type or individual) is instance of this type; any type is also a subtype of this type
                 supertype:  divisible_thing__divisiblething
                    supertype:  thing__something___T__t___3D_or_4D_thing_or_anything_else  any category (type or individual) is instance of this type; any type is also a subtype of this type

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