/* examples to illustrate controlling search in CLP(R) */

/* append with the rules reversed */

append([X|Xs],Ys,[X|Zs]) :- append(Xs,Ys,Zs).
append([],Ys,Ys).

/* length of a list, without and with a (usually redundant) constraint that
   the length is non-negative */

not_so_good_length([],0).
not_so_good_length([_|Xs],N) :- not_so_good_length(Xs,N-1).

length([],0).
length([_|Xs],N) :- N>0, length(Xs,N-1).

/* the standard definition of member */
member(X,[X|Xs]).
member(X,[Y|Ys]) :- member(X,Ys).

/* cut (written !) prunes the search tree */

member_cut(X,[X|Xs]) :- !.
member_cut(X,[Y|Ys]) :- member_cut(X,Ys).

/*

member(1,[1,2,3,1])   will succeed twice

member_cut(1,[1,2,3,1])   will succeed just once

member_cut(X,[1,2,3]) will only give one answer: X=1.

*/

my_not(X) :- call(X), !, fail.
my_not(X).