
[;1m  family_union(Family1, Family2)[0m

  If [;;4mFamily1[0m and [;;4mFamily2[0m are families, then [;;4mFamily3[0m is the
  family such that the index set is the union of [;;4mFamily1[0m:s and [;;4m[0m
  [;;4mFamily2[0m:s index sets, and [;;4mFamily3[0m[i] is the union of [;;4mFamily1[0m
  [i] and [;;4mFamily2[0m[i] if both map i, otherwise [;;4mFamily1[0m[i] or [;;4m[0m
  [;;4mFamily2[0m[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]}]
