====== Predecessor ======
{{tag>family recursion}}
===== Description =====
Four versions of the predecessor program.

**Source**:  PROLOG programming for artificial intelligence, 3rd Edition, Harlow, 2001, ISBN 0-201-40375-7.
===== Download =====
Program source code: {{predecessor.pl}}
===== Listing =====
<code prolog>
% 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).



</code>
===== Comments =====