P ->  | R or P -> ; R

     Analogous to

                                 "if P then  else R"

     i.e. defined as if by

     P -> Q | R :- P, !, Q.
     P -> Q | R :- R.


P -> Q

   When  occurring  other  than  as  one  of  the  alternatives  of  a
   disjunction, is equivalent to:

     P ->  | fail.
