hoge.lisp

      (let* ((p1 b)
	     (v1 (v- a b))
	     (p2 d)
	     (v2 (v- c d)))
	(with-vector (x1 y1 z1) v1
	  (with-vector (x2 y2 z2) v2
	    (with-vector (x3 y3 z3) (v- p1 p2)
	      (when (<= (abs (- (+ (* x1 y2 z3) (* x2 y3 z1) (* x3 y1 z2))
				(+ (* x3 y2 z1) (* y3 z2 x1) (* z3 x2 y1))))
			*vector3-acceptable-error-range*)
		(let ((d (- (* x1 y2) (* x2 y1))))
		  (break)
		  (if (not (zerop d))
		      (let ((l1 (- (/ (- (* x3 y2) (* y3 x2)) d)))
			    (l2 (/ (- (* x1 y3) (* y1 x3)) d)))
			(break)
			(if (and (<= 0 l1 1)
				 (<= 0 l2 1))
			    (v+ p1 (v* v1 l1))))))))))))))