(^)/2 operator is too defensive

[ghost]

SWI-Prolog can compute 9^(9^9) without a resource error and
very quickly ( :-) ), even if despite it has ca. 370 million digits:

   Welcome to SWI-Prolog (threaded, 64 bits, version 7.7.15)
   SWI-Prolog comes with ABSOLUTELY NO WARRANTY. This is free software.

   ?- [user].
   pow(_,0,1) :- !.
   pow(X,1,X) :- !.
   pow(X,N,Y) :-
        M is N//2, pow(X,M,A),
        J is N mod 2, pow(X,J,B),
        Y is (A*A)*B.
   ^D

   ?- pow(9,9,X).
   X = 387420489.

   ?- time((pow(9,9,X), pow(9,X,_))).
   % 159 inferences, 7.500 CPU in 8.514 seconds (88% CPU, 21 Lips)

[JanWielemaker]

Thanks. Fixed with 5af3226