(bind-func add-head
(lambda (head:Vector4* w density)
(let ((x (ftod (tref head 0)))
(y (ftod (tref head 1))))
(add-density-xy x y density)
(add-density-xy (- x w) y density)
(add-density-xy (+ x w) y density)
(add-density-xy x (- y w) density)
(add-density-xy x (+ y w) density)
void)))