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

  Returns the restriction of the binary relation BinRel1 to Set.

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