
  [;1m-spec union_of_family(Family) -> Set[0m
  [;1m                         when Family :: family(), Set :: a_set().[0m

  Returns the union of family [;;4mFamily[0m.

    1> F = sofs:family([{a,[0,2,4]},{b,[0,1,2]},{c,[2,3]}]),
    S = sofs:union_of_family(F),
    sofs:to_external(S).
    [0,1,2,3,4]
