/* We use the two triangles in the parallelogram vb, vb+e5, vb+e0, vb+e1.
* We go round each triangle clockwise (although our surface is non-
* orientable so it shouldn't matter). Picking the parallelogram
/* We use the two triangles in the parallelogram vb, vb+e5, vb+e0, vb+e1.
* We go round each triangle clockwise (although our surface is non-
* orientable so it shouldn't matter). Picking the parallelogram