a few words on cut:

!

(type a cut as !)

*   ! is a goal that always succeeds, BUT has side effects that
    prevent it from being able to be backtracked over;

Clocksin and Mellish give 3 "common uses" of cut:
   (pp. 64-78)

   *   first - "we want to tell the Prolog system that it has found
       the RIGHT rule for a particular goal"

       says: "if you get this far, you have picked the CORRECT rule for
       this goal"

   *   second (when cut is used with fail/0): 
       "we want to tell the Prolog system to fail a particular
       goal immediately, WITHOUT trying for alternative solutions"

       says: "if you get to here, you should STOP trying to satisfy this
       goal"

   *   third: "we want to terminate the generation of alternative
       solutions through backtracking"

       says: "if you get to here, you have found the only solution
       to this problem, and there is no point in ever looking for
       alternatives"

foo :- a, b, c, !, d, e, f.

(source: Pomona tutorial)

*   green cut: a cut that is JUST to improve efficiency
*   red cut: to restrict answers; to block what what would
    otherwise be reported as consequences of the program