Prolog maximum works like this:
			?- R is max(3,4).
			R = 4
	See below our versions. Which one do you prefer? 

			max(X,Y,X):- X >= Y.
          	max(X,Y,Y):- Y > X.         
    	  	?- max(5,3,R).
          	R = 5
          	no

          	maxx(X,Y,X):- X >= Y.
          	maxx(X,Y,Y).
          	?- maxx(3,5,R).
          	R = 5
          	no
          	?- maxx(5,3,R).
          	R = 5
          	R = 3

          	maxxx(X,Y,X):- X >= Y,!.
          	maxxx(X,Y,Y).
          	?- maxxx(5,3,R).
          	R = 5
          	no