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dekker.one
1predicate fzn_tree(array[int] of $$N: from, array[int] of $$N: to,
2 var $$N: r, array[$$N] of var bool: ns, array[int] of var bool: es) =
3 let {
4 int: E = length(es);
5 array[1..2*E] of int: dfrom = from ++ to;
6 array[1..2*E] of int: dto = to ++ from;
7 array[1..2*E] of var bool: des;
8 } in
9 /* ensure that the directed edges selected agree with undirected edges */
10 forall(e in 1..E)(es[e-1+min(index_set(es))] <-> (des[e] \/ des[e+E])) /\
11 /* duplicate the edges so that the we can use directed graph reachability */
12 fzn_dtree(dfrom,dto,r,ns,des);
13
14%-----------------------------------------------------------------------------%