
  -spec canonical_relation(SetOfSets) -> BinRel
                              when
                                  BinRel :: binary_relation(),
                                  SetOfSets :: set_of_sets().

  Returns the binary relation containing the elements (E, Set) such
  that Set belongs to SetOfSets and E belongs to Set. If 
  SetOfSets is a partition of a set X and R is the equivalence
  relation in X induced by SetOfSets, then the returned relation
  is the canonical map from X onto the equivalence classes with
  respect to R.

    1> Ss = sofs:from_term([[a,b],[b,c]]),
    CR = sofs:canonical_relation(Ss),
    sofs:to_external(CR).
    [{a,[a,b]},{b,[a,b]},{b,[b,c]},{c,[b,c]}]
