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

  If Family1 is a family and Family1[i] is a binary relation
  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 domain of Family1[i].

    1> FR = sofs:from_term([{a,[{1,a},{2,b},{3,c}]},{b,[]},{c,[{4,d},{5,e}]}]),
    F = sofs:family_domain(FR),
    sofs:to_external(F).
    [{a,[1,2,3]},{b,[]},{c,[4,5]}]
