if (an == dn) {
/* We have a match; so (WLOG) the 'A' marked above are at
* (x1,y1) and (x2,y2), and the 'B' are at (ax,ay) and (dx,dy). */
if (an == dn) {
/* We have a match; so (WLOG) the 'A' marked above are at
* (x1,y1) and (x2,y2), and the 'B' are at (ax,ay) and (dx,dy). */