
  [;1m-spec drestriction(BinRel1, Set) -> BinRel2[0m
  [;1m                      when[0m
  [;1m                          BinRel1 :: binary_relation(),[0m
  [;1m                          BinRel2 :: binary_relation(),[0m
  [;1m                          Set :: a_set().[0m

  Returns the difference between the binary relation [;;4mBinRel1[0m and
  the restriction of [;;4mBinRel1[0m to [;;4mSet[0m.

    1> R1 = sofs:relation([{1,a},{2,b},{3,c}]),
    S = sofs:set([2,4,6]),
    R2 = sofs:drestriction(R1, S),
    sofs:to_external(R2).
    [{1,a},{3,c}]

  [;;4mdrestriction(R, S)[0m is equivalent to [;;4m[0m
  [;;4mdifference(R, restriction(R, S))[0m.
