anchor.diff:  		(when (= (slot-value chunk 'references) 0)
   			(unload-chunk chunk-x chunk-y))))
   
  +(defun require-gravity-area (chunk-x chunk-y)
  +	;; Go through every chunk in the gravity area
  +	;; x and y will take values from -*gravity-area-size* to *gravity-area-size*, inclusive
  +	(do ((x (- *gravity-area-size*) (+ x 1)))
  +	    ((> x *gravity-area-size*))
  +		(do ((y (- *gravity-area-size*) (+ y 1)))
  +		    ((> y *gravity-area-size*))
  +			;; Ignore the chunk in the middle (it's handled otherwise)
  +			(unless (and (= x 0) (= y 0))
  +				(require-chunk x y)))))
  +
  +(defun unrequire-gravity-area (chunk-x chunk-y)
  +	;; Go through every chunk in the gravity area
  +	;; x and y will take values from -*gravity-area-size* to *gravity-area-size*, inclusive
  +	(do ((x (- *gravity-area-size*) (+ x 1)))
  +	    ((> x *gravity-area-size*))
  +		(do ((y (- *gravity-area-size*) (+ y 1)))
  +		    ((> y *gravity-area-size*))
  +			;; Ignore the chunk in the middle (it's handled otherwise)
  +			(unless (and (= x 0) (= y 0))
  +				(unrequire-chunk x y)))))
  +
   (defun enter-chunk (ship chunk-x chunk-y)
   	;; Mark that we use this chunk
   	(require-chunk chunk-x chunk-y)
  +	;; Mark gravity area also as used
  +	(require-gravity-area chunk-x chunk-y)
   	(let ((chunk (get-chunk chunk-x chunk-y)))
   		;; Add ship to the list of ships
   		(setf (slot-value chunk 'ships) (cons ship (slot-value chunk 'ships)))))
  
positive.code: (defvar *space* (make-hash-table :test 'equal))
  (defparameter *chunk-width* 256)
  (defparameter *chunk-height* 256)
  
  (defun generate-chunk (chunk-x chunk-y)
  	;; TODO: actually generate something
  
negative.code: 		;; Remove ship from the list of ships
  		(setf (slot-value chunk 'ships) (remove ship (slot-value chunk 'ships))))
  	;; Mark that we no longer use this chunk
  	(unrequire-chunk chunk-x chunk-y)
  	;; Mark that we no longer use the gravity area either
  	(unrequire-gravity-area chunk-x chunk-y))
  
  (defun ships-in-chunk (chunk-x chunk-y)
  	(let ((chunk (get-chunk chunk-x chunk-y)))
  