introduced by chaining operations. When two ViewEdges are concatenated by a chaining operator, the last vertex of one ViewEdge and the first vertex of the other reside in the same 3D position, so that the latter vertex is omitted. This caused a pair of SVertices unconnected by an FEdge. The present commit intends to fix this issue. The bug was reported by mato_sus304 with a .blend file reproducing the issue. Thanks!