1% fact for each vertex(N).
    2vertex(0).
    3vertex(1).
    4vertex(2).
    5
    6% fact for each edge edge(U, V).
    7edge(0, 1).
    8edge(1, 2).
    9edge(2, 0).
   10
   11reachable(V) :- chosen(U, V), reachable(U).
   12reachable(0) :- chosen(V, 0).
   13
   14% Every vertex must be reachable.
   15:- vertex(U), not reachable(U).
   16
   17% Choose exactly one edge from each vertex.
   18other(U, V) :-
   19    vertex(U), vertex(V), vertex(W),
   20    edge(U, W), V \= W, chosen(U, W).
   21chosen(U, V) :-
   22    edge(U, V), not other(U, V).
   23
   24% You cannot choose two edges to the same vertex
   25:- chosen(U, W), chosen(V, W), U \= V.   26
   27?- chosen(1,2).