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%   Example code from the book "Natural Language Processing in Prolog"  %
%                      published by Addison Wesley                      %
%        Copyright (c) 1989, Gerald Gazdar & Christopher Mellish.       %
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%
% xx.dcg [Chapter  4] A DCG for the language xx, x in {a,b,c}*
%
% put first character onto the stack
x	-->	[a],	x(a).
x	-->	[b],	x(b).
x	-->	[c],	x(c).
% push subsequent characters onto stack
x(I)	-->	[a],	x(a(I)).
x(I)	-->	[b],	x(b(I)).
x(I)	-->	[c],	x(c(I)).
% copy stack to a new nonterminal
x(I)	-->	y(I).
% pop the characters on the top of the stack
y(a(I))	-->	y(I),	[a].
y(b(I))	-->	y(I),	[b].
y(c(I))	-->	y(I),	[c].
% pop the final character
y(a)	-->	[a].
y(b)	-->	[b].
y(c)	-->	[c].