
  -spec relative_product1(BinRel1, BinRel2) -> BinRel3
                             when
                                 BinRel1 :: binary_relation(),
                                 BinRel2 :: binary_relation(),
                                 BinRel3 :: binary_relation().

  Returns the relative product of the converse of the binary
  relation BinRel1 and the binary relation BinRel2.

    1> R1 = sofs:relation([{1,a},{1,aa},{2,b}]),
    R2 = sofs:relation([{1,u},{2,v},{3,c}]),
    R3 = sofs:relative_product1(R1, R2),
    sofs:to_external(R3).
    [{a,u},{aa,u},{b,v}]

  relative_product1(R1, R2) is equivalent to 
  relative_product(converse(R1), R2).
