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

  If [;;4mFamily1[0m is a family, then [;;4mFamily2[0m is the family with the
  same index set as [;;4mFamily1[0m such that [;;4mFamily2[0m[i] is the result
  of calling [;;4mSetFun[0m with [;;4mFamily1[0m[i] as argument.

    1> F1 = sofs:from_term([{a,[[1,2],[2,3]]},{b,[[]]}]),
    F2 = sofs:family_projection(fun sofs:union/1, F1),
    sofs:to_external(F2).
    [{a,[1,2,3]},{b,[]}]
