• Let c be the first vertex category such that there is a triplet of non-coplanar edges starting from the same vertex in c (if no such category exists, skip to the alternative below). Then, for all such triplets, let c 1 , c 2 and c 3 be the sorted categories of the vertices to which the initial vertex is bonded with these edges. There are four subsets of such triplets, characterised by whether they satisfy: