scratch the above comment! it's panicing because it's rightfully OOM when it can't store 10000 geojson objects in 8M heap.

good news though, we can strip nonblocking-input-ports and still reproduce it:

;;; running like this:
;;; [klm@kth arp]$ csi -s geojson-generator.scm | csi -s ./arp-boats.scm -:g -:hm8M 2>&1 | grep resize -C 5

(use cjson)

(define state '())

(define (geojson-mmsi geojson)
  (let ((mmsi (alist-ref 'mmsi (or (alist-ref 'properties geojson) '()))))
    (assert (number? mmsi))
    mmsi))

(define (prj-geojson geojson state)
  (assert (<= (length state) 20000))
  ;; set properties for mmsi key:
  (alist-update (geojson-mmsi geojson) ;; key (mmsi)
                geojson ;; value (geojson feature)
                state))

(define count 0)
(port-for-each
 (lambda (line)
   (let ((json (string->json line)))
     (set! count (add1 count))
     (set! state (prj-geojson json state))))
 read-line)