
[;1m  specification(Fun, Set1)[0m

  Returns the set containing every element of [;;4mSet1[0m for which [;;4mFun[0m
  returns [;;4mtrue[0m. If [;;4mFun[0m is a tuple [;;4m{external, Fun2}[0m, [;;4mFun2[0m is
  applied to the external set of each element, otherwise [;;4mFun[0m is
  applied to each element.

    1> R1 = sofs:relation([{a,1},{b,2}]),
    R2 = sofs:relation([{x,1},{x,2},{y,3}]),
    S1 = sofs:from_sets([R1,R2]),
    S2 = sofs:specification(fun sofs:is_a_function/1, S1),
    sofs:to_external(S2).
    [[{a,1},{b,2}]]
