(defun corner-p (sq) (assoc sq corner/xsqs))
(defun x-square-p (sq) (rassoc sq corner/xsqs))
(defun x-square-for (corner) (cdr (assoc corner corner/xsqs)))
(defun corner-p (sq) (assoc sq corner/xsqs))
(defun x-square-p (sq) (rassoc sq corner/xsqs))
(defun x-square-for (corner) (cdr (assoc corner corner/xsqs)))