(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)) )))