Four versions of the predecessor program.

Source: PROLOG programming for artificial intelligence, 3rd Edition, Harlow, 2001, ISBN 0-201-40375-7.

Program source code: predecessor.pl

% Figure 2.16   Four versions of the predecessor program.
 
 
% Four versions of the predecessor program
 
% The original version
 
pred1( X, Z)  :-
   parent( X, Z).
 
pred1( X, Z)  :-
   parent( X, Y),
   pred1( Y, Z).
 
% Variation a: swap clauses of the original version
 
pred2( X, Z)  :-
   parent( X, Y),
   pred2( Y, Z).
 
pred2( X, Z)  :-
   parent( X, Z).
 
% Variation b: swap goals in second clause of the original version
 
pred3( X, Z)  :-
   parent( X, Z).
 
pred3( X, Z)  :-
   pred3( X, Y),
   parent( Y, Z).
 
% Variation c: swap goals and clauses of the original version
 
pred4( X, Z)  :-
   pred4( X, Y),
   parent( Y, Z).
 
pred4( X, Z)  :-
   parent( X, Z).