
  -spec family_union(Family1, Family2) -> Family3
                        when
                            Family1 :: family(),
                            Family2 :: family(),
                            Family3 :: family().

  If Family1 and Family2 are families, then Family3 is the
  family such that the index set is the union of Family1:s and 
  Family2:s index sets, and Family3[i] is the union of Family1
  [i] and Family2[i] if both map i, otherwise Family1[i] or 
  Family2[i].

    1> F1 = sofs:family([{a,[1,2]},{b,[3,4]},{c,[5,6]}]),
    F2 = sofs:family([{b,[4,5]},{c,[7,8]},{d,[9,10]}]),
    F3 = sofs:family_union(F1, F2),
    sofs:to_external(F3).
    [{a,[1,2]},{b,[3,4,5]},{c,[5,6,7,8]},{d,[9,10]}]
