// (Not sure which is head or tail, or which hand the normal lies)
// [v1 should be the tail, v2 should be the head, in which case the normal should
// rotate anti-clockwise.]
// (Not sure which is head or tail, or which hand the normal lies)
// [v1 should be the tail, v2 should be the head, in which case the normal should
// rotate anti-clockwise.]