/* SYS$USER2:[PROLOG.PRO]LISTS.DAT;1 */

/* -------------------------- */
/*  some list processing....  */
/* -------------------------- */

/*  append  */
/*  append(List1, List2, List3) -> "List3 <-- List1 + List2"  */
append([],L,L).
append([H | T],L,[H | V]) :- append(T,L,V).

/*  last  */
/*  last(X, List) -> "X is the last element in List"  */
last(X,[X]).
last(X,[_|Y]) :- last(X,Y).

/*  nextto  */
/*  nextto(X,Y,List) -> "X and Y are consecutive elements in List"  */
nextto(X,Y,[X,Y|_]).
nextto(X,Y,[_|Z]) :- nextto(X,Y,Z).

/*  reverse  */
/*  reverse(List1, List2) -> "List2 is the reversing order of List1" */
reverse([],[]).
reverse([H|T],List) :- reverse(T,Z), append(Z,[H],List).

/*  efface  */
/*  efface(X,List1,List2) -> "Delete the first occurrence           */
/*                            of X from List1 to get List2"         */
efface(_,[],[]).
efface(X,[X|L],L) :- !.
efface(X,[Y|L],[Y|M]) :- efface(X,L,M).

/*  delete  */
/*  delete(X, List1, List2) -> "delete all the elements X from List1 to get List2*/
delete(_,[],[]).
delete(X,[X|L],M) :- !, delete(X,L,M).
delete(X,[Y|L1],[Y|L2]) :- delete(X,L1,L2).

/*  subst  */

/* subst(X,List1,A,List2©  -¾  "replaceå all X in List 1 by A to get List2*/
subst(D1,[],D2,[]).
subst(X,[X|L],A,[A|M]) :- !, subst(X,L,A,M).
subst(X,[Y|L],A,[Y|M]) :- subst(X,L,A,M).

/*  sublist  */
/*  sublist(S,List) -> "S is a sublist of List which appears consecutively,  */
/*                      and in the same order"                               */
sublist([X|L],[X|M]) :- prefix(L,M).
sublist(L,[_|M]) :- sublist(L,M).

prefix([],_).
prefix([X|L],[X|M]) :- prefix(L,M).

/* --------- */
/*  testing  */
/* --------- */
?- last(X,[talk,of,the,town]), !.
?- last(town,[talk,of,the,town]), !.
?- last(of,[talk,of,the,town]), !.

?- nextto(X,Y,[a,b,c,d,e,f,g,h]), !.
?- nextto(d,Y,[a,b,c,d,e,f,g,h]), !.
?- nextto(X,g,[a,b,c,d,e,f,g,h]), !.
?- nextto(e,f,[a,b,c,d,e,f,g,h]), !.
?- nextto(b,g,[a,b,c,d,e,f,g,h]), !.

?- append([a,b,c],[d,e,f],List), !.
?- append(X,Y,[a,b,c,d]), !.
?- append(X,[d,e,f],[a,b,c,d,e,f]), !.
?- append([a,b,c],Y,[a,b,c,d,e,f]), !.

?- reverse([a,b,c,d,e],L), !.
?- reverse(L,[a,b,c,d,e]), !.

?- efface(s,[a,y,s,g,e,s,o,s],L), !.
?- efface(X,[h,e,e,l],L), !.
?- efface(X, [h,e,e,l], [h,e,l]), !.

?- delete(s,[s,o,s,s,y,s],L), !.
?- delete(X,[s,o,s,s,y,s],L), !.
?- subst(a,[a,b,b,a,u],i,L), !.

?- sublist([of,the,club],[meeting,of,the,club,shall,be,called]), !.
?- sublist([be,the,club],[meeting,of,the,club,shall,be,called]), !.