
  -spec family_intersection(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 intersection of Family1:s
  and Family2:s index sets, and Family3[i] is the intersection
  of Family1[i] and 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_intersection(F1, F2),
    sofs:to_external(F3).
    [{b,[4]},{c,[]}]
