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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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%
% mix3.dcg [Chapter  4] A DCG for the language MIX, MIX in {a,b,c}*, n(a) = n(b) = n(c)
%
% start three counters
x		-->	x(a,b,c).
% if there is an a, then increment the b and c counters
x(A,B,C)	-->	[a],	x(A,b(B),c(C)).
x(A,B,C)	-->	[b],	x(a(A),B,c(C)).
x(A,B,C)	-->	[c],	x(a(A),b(B),C).
% alternatively, if there is an a, then decrement the a counter
x(a(A),B,C)	-->	[a],	x(A,B,C).
x(A,b(B),C)	-->	[b],	x(A,B,C).
x(A,B,c(C))	-->	[c],	x(A,B,C).
% eliminate the nonterminal
x(a,b,c)	-->	[].