
  -spec family_union(Family1) -> Family2
                        when Family1 :: family(), Family2 :: family().

  If Family1 is a family and Family1[i] is a set of sets for
  each i in the index set of Family1, then Family2 is the family
  with the same index set as Family1 such that Family2[i] is the 
  union of Family1[i].

    1> F1 = sofs:from_term([{a,[[1,2],[2,3]]},{b,[[]]}]),
    F2 = sofs:family_union(F1),
    sofs:to_external(F2).
    [{a,[1,2,3]},{b,[]}]

  family_union(F) is equivalent to family_projection(fun
  sofs:union/1, F).
