
  [;1m-spec family_field(Family1) -> Family2[0m
  [;1m                      when Family1 :: family(), Family2 :: family().[0m

  If [;;4mFamily1[0m is a family and [;;4mFamily1[0m[i] is a binary relation
  for every i in the index set of [;;4mFamily1[0m, then [;;4mFamily2[0m is the
  family with the same index set as [;;4mFamily1[0m such that [;;4mFamily2[0m[i]
  is the field of [;;4mFamily1[0m[i].

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

  [;;4mfamily_field(Family1)[0m is equivalent to [;;4m[0m
  [;;4mfamily_union(family_domain(Family1), family_range(Family1))[0m.
