anchor.diff:      (with-open-file (in pathname :if-does-not-exist if-does-not-exist)
         (values-list (read in)))))
   
  -(defun perform-program (data-pathname depth leaf &rest args &key &allow-other-keys)
  -  (multiple-value-bind (area tree) (read-program-data data-pathname)
  -    (let ((chains (compute-chains-to-depth tree depth)))
  -      (apply #'perform-leaf area (elt chains leaf) args))))
  +(defun perform-program (data-pathname depth leaf &rest args &key chains-group-size chains-verbose &allow-other-keys)
  +  (alexandria:remove-from-plistf args :chains-group-size)
  +  (let* ((chains-group-size (or chains-group-size 1)))
  +    (multiple-value-bind (area tree) (read-program-data data-pathname)
  +      (let ((chains (compute-chains-to-depth tree depth)))
  +	(loop
  +	   with number-of-chains = (length chains)
  +	   for index from (* leaf chains-group-size) below (min number-of-chains (* (1+ leaf) chains-group-size))
  +	   do
  +	     (when chains-verbose
  +	       (format t "~&;;;; Starting leaf ~d at depth ~d~%" index depth))
  +	     (handler-case (apply #'perform-leaf area (elt chains index) args)
  +	       (error (c)
  +		 (format *error-output* "~&;;;; Encountered error whilst executing leaf ~d for depth ~d:~%~A~%" index depth c))))))))
   
   (defun break-string (string)
     (split-sequence:split-sequence #\Space string))
  
positive.code: ~A
  "
  	  (print-program-usage/option-text '(("help" nil "This helpful message.")
  					     ("force" nil "Overwrite any existing output.")))
  	  (print-program-usage/suffix)))
  
  (defun print-program-usage (help-data &optional (stream *standard-output*))
  
negative.code: ~A
  "
  	  (print-program-usage/option-text '(("help" nil "This helpful message.")
  					     ("force" nil "Overwrite any existing output.")
  					     ("chains-verbose" nil "Output information which leaf is being executed.")
  					     ("chains-group-size" "size" "Execute <size> leaves starting at <leaf>*<size>.")))
  	  (print-program-usage/option-text help-data)
  	  (print-program-usage/suffix)))
  
  