no title added by anonymous on Wed Oct 25 22:57:44 2017
(let ((stopi w)) (let loopi ((i starti))
(unless (>= i stopi)
(let ((stopj h)) (let loopj ((j startj))
(if (or (>= j stopj) (= (array-ref arr i j) particle-type) )
(cons i j)
(loopj (inc j)) ) ))
(loopi (inc i)) )))