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

  Returns a subset of [;;4mSet1[0m containing those elements that do not
  give an element in [;;4mSet2[0m as the result of applying [;;4mSetFun[0m.

    1> SetFun = {external, fun({_A,B,C}) -> {B,C} end},
    R1 = sofs:relation([{a,aa,1},{b,bb,2},{c,cc,3}]),
    R2 = sofs:relation([{bb,2},{cc,3},{dd,4}]),
    R3 = sofs:drestriction(SetFun, R1, R2),
    sofs:to_external(R3).
    [{a,aa,1}]

  [;;4mdrestriction(F, S1, S2)[0m is equivalent to [;;4m[0m
  [;;4mdifference(S1, restriction(F, S1, S2))[0m.
