
  -spec family_difference(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 equal to the index set of 
  Family1, and Family3[i] is the difference between Family1[i]
  and Family2[i] if Family2 maps i, otherwise Family1[i].

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