%%% Difference lists --- see p&S p.126-7

conc_dl(Front-Back1, Back1-Back2, Front-Back2).

%| ?- [conc_dl].
%% consulting /amd/nfs/pegasus/disk/ptn053/steedman/lectures/tl/conc_dl...
%% consulted /amd/nfs/pegasus/disk/ptn053/steedman/lectures/tl/conc_dl in module user, 0 msec -88 bytes
%yes
%| ?- conc_dl([1,2,3|X]-X, [4,5,6|Y]-Y, R).
%R = [1,2,3,4,5,6|Y]-Y,
%X = [4,5,6|Y] ? 
%yes
%| ?- 

%%% Reflect that S -> NP VP is equivalent to saying
%%% s(W) :- np(U), vp(V), conc_dl(U,V,W).
%%% 
%%% This can be partially executed to get rid of the last literal by
%%% writing
%%% s(P0-P) :- np(P0-P1), vp(P1-P).
%%% 
%%% The following clause then induces a representation of string position
%%% in terms of the syubstring following the position.  

connects(Word, [Word|Rest],Rest).

% | ?- [dcg0].
%% consulting /amd/nfs/pegasus/disk/ptn053/steedman/lectures/tl/dcg0...
%% consulted /amd/nfs/pegasus/disk/ptn053/steedman/lectures/tl/dcg0 in module user, 0 msec -88 bytes
%yes
%| ?- [conc_dl].
%% consulting /amd/nfs/pegasus/disk/ptn053/steedman/lectures/tl/conc_dl...
%The procedure connects/3 is being redefined.
%    Old file: /amd/nfs/pegasus/disk/ptn053/steedman/lectures/tl/dcg0
%    New file: /amd/nfs/pegasus/disk/ptn053/steedman/lectures/tl/conc_dl
%Do you really want to redefine it?  (y, n, p, s, a, b, or ?) y
%% consulted /amd/nfs/pegasus/disk/ptn053/steedman/lectures/tl/conc_dl in module user, 0 msec -488 bytes
%yes
%| ?- s([harry,sees,the,frog,that,walks],[]).
%yes
%| ?- s([harry,sees,the,frog,that],[]).
%no
%| ?- s([harry,sees,the,frog],[]).
%yes
%| ?- 
%
%%%% Running in (-,+) mode
%| ?- s(L,[]).
%L = [harry,sees,harry] ? ;
%L = [harry,sees,barry] ? ;
%L = [harry,sees,the,frog] ? ;
%L = [harry,sees,the,frog,that,sees,harry] ? ;
%L = [harry,sees,the,frog,that,sees,barry] ? ;
%L = [harry,sees,the,frog,that,sees,the,frog] ? ;
%L = [harry,sees,the,frog,that,sees,the,frog,that,sees|...] ? ;
%L = [harry,sees,the,frog,that,sees,the,frog,that,sees|...] ? 
%yes
%| ?- 
%%%% NB we never get e.g the frog sees harry.