anchor.diff:                  (draw-line-strokes pane view pump-state start-offset cursor-x cursor-y)
                 (setf pump-state new-pump-state
                       start-offset (1+ (line-end-offset line)))
  -              (incf cursor-y line-height))
  +              (incf cursor-y (+ line-height (stream-vertical-spacing pane))))
            when (or (>= (y2 (line-dimensions line)) pane-height)
                     (= (line-end-offset line) (size (buffer view))))
            return (progn
  
positive.code: 
  (defstruct (dimensions :conc-name)
    "A simple mutable rectangle structure. The coordinates should
  be absolute coordinates in the coordinate system of a sheet."
    (x1 0)
    (y1 0)
    (x2 0)
    (y2 0))
  
  (defun dimensions-height (dimensions)
    "Return the width of the provided `dimensions' object."
  
negative.code:       (incf (line-stroke-count line))
        (setf (line-end-offset line) (stroke-end-offset stroke)))))
  
  (defun record-stroke (stroke x1 y1 x2 y2
                        &optional (center (/ (- y2 y1) 2)))
    "Record the fact that `stroke' has been drawn, and that it
  covers the specified area on screen. Updates the dirty- and
  modified-bits of `stroke' as well as the dimensions."
  