
  -spec range(BinRel) -> Set
                 when BinRel :: binary_relation(), Set :: a_set().

  Returns the range of the binary relation BinRel.

    1> R = sofs:relation([{1,a},{1,b},{2,b},{2,c}]),
    S = sofs:range(R),
    sofs:to_external(S).
    [a,b,c]
