
  -spec relation_to_family(BinRel) -> Family
                              when
                                  Family :: family(),
                                  BinRel :: binary_relation().

  Returns family Family such that the index set is equal to the 
  domain of the binary relation BinRel, and Family[i] is the 
  image of the set of i under BinRel.

    1> R = sofs:relation([{b,1},{c,2},{c,3}]),
    F = sofs:relation_to_family(R),
    sofs:to_external(F).
    [{b,[1]},{c,[2,3]}]
