It is helpful to you in writing, understanding and correcting your programs if you keep similar objects together. The path facts have been ordered by the numerical order of their arguments.

     path(2,5).

     path(3,4).
     path(3,7).

     path(4,5).

     path(5,8).

     path(6,7).
     path(6,9).

     path(7,b).
     path(7,8).

     path(8,b).

     path(9,b).