
  -spec drestriction(BinRel1, Set) -> BinRel2
                        when
                            BinRel1 :: binary_relation(),
                            BinRel2 :: binary_relation(),
                            Set :: a_set().

  Returns the difference between the binary relation BinRel1 and
  the restriction of BinRel1 to Set.

    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}]

  drestriction(R, S) is equivalent to 
  difference(R, restriction(R, S)).
