If there was no such pre-candidate, the candidates would have been all the precandidates where c 1 = c 2 with c 1 minimal, i.e. "filled" if possible, "empty" otherwise. • Alternatively, if all vertices have only coplanar outgoing edges, let c be the first category such that there exists another category c ′ such that there is a non-coplanar triplet of edges, the two first of which start from a vertex of c and the last from c ′ . c ′ is chosen as the first such category. Let c 1 , and c 2 be the sorted categories of the destinations of the two first vertices, and c 3 be the destination of the third. Then the candidates are all the triplets of edges, the two first of which start in c and the last from c ′ such that (c 1 , c 2 , c 3 ) is lexicographically minimal.