
  -spec extension(BinRel1, Set, AnySet) -> BinRel2
                     when
                         AnySet :: anyset(),
                         BinRel1 :: binary_relation(),
                         BinRel2 :: binary_relation(),
                         Set :: a_set().

  Returns the extension of BinRel1 such that for each element E
  in Set that does not belong to the domain of BinRel1, 
  BinRel2 contains the pair (E, AnySet).

    1> S = sofs:set([b,c]),
    A = sofs:empty_set(),
    R = sofs:family([{a,[1,2]},{b,[3]}]),
    X = sofs:extension(R, S, A),
    sofs:to_external(X).
    [{a,[1,2]},{b,[3]},{c,[]}]
