for every edge, for each of its 1 or 2 adjacent faces, a +1 or -1 for each face indicating which side of the edge the face lies on.
An edge is adjacent to one or two faces, this is stored in its edges_faces entry. for each of these faces, which side of the edge (using the edge's vector) is each face on. each result is an array of length matching the number of adjacent edges.
the fold object with edges_faces and faces_center