% Example of a tight program
% (c) Tomi Janhunen
%
% The stable and supported models of the program coincide
% usage: lparse -cn=<number of nodes> tight.lp | smodels 0
% or     lparse -cn=<number of nodes> tight.lp | support | smodels 0

node(0..n-1).

% Define a ring of edges

edge(X,(X+1) mod n) :- node(X).

hide node(X).
hide edge(X,Y).

% Choose edges which are "in" and which "out"

in(X,Y) :- not out(X,Y), edge(X,Y).
out(X,Y) :- not in(X,Y), edge(X,Y).

% Constraints (exactly one edge is "out" others "in")

:- in(X,Y):edge(X,Y).
:- out(X,Y), out(Z,V), X!=Z, edge(X,Y;Z,V).

% Reachability over edges that are "in"
	
reach(X,Y) :- in(X,Y), edge(X,Y).
reach(X,Y) :- reach(X,Z), in(Z,Y), node(X), edge(Z,Y).