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

  If Family1 is a family and Family1[i] is a set of sets for
  every 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 intersection of Family1[i].

  If Family1[i] is an empty set for some i, the process exits with
  a badarg message.

    1> F1 = sofs:from_term([{a,[[1,2,3],[2,3,4]]},{b,[[x,y,z],[x,y]]}]),
    F2 = sofs:family_intersection(F1),
    sofs:to_external(F2).
    [{a,[2,3]},{b,[x,y]}]
