
[;1m  restriction(SetFun, Set1, Set2)[0m

  Returns a subset of [;;4mSet1[0m containing those elements that gives an
  element in [;;4mSet2[0m as the result of applying [;;4mSetFun[0m.

    1> S1 = sofs:relation([{1,a},{2,b},{3,c}]),
    S2 = sofs:set([b,c,d]),
    S3 = sofs:restriction(2, S1, S2),
    sofs:to_external(S3).
    [{2,b},{3,c}]
