anchor.diff:  (define-command toggle-mark ()
     "Toggle the mark."
     (let ((mode (find-submode (current-buffer) 'visual-mode)))
  -    (if (string= (is-collapsed) "true")
  +    (if (collapsed-p)
           (progn
             (setf (mark-set mode) (not (mark-set mode)))
             (if (mark-set mode)
  
positive.code: strong, sub, sup, listing, xmp, plaintext, basefont, big, blink, center, font,
  marquee, multicol, nobr, s, spacer, strike, tt, u, wbr, code, cite, pre"))
  
  (define-parenscript is-collapsed ()
    ;; returns "true" if mark's start and end are the same value
    (defun is-collapsed ()
      (let ((sel (ps:chain window (get-selection))))
        (ps:@ sel is-collapsed)))
    (is-collapsed))
  
  (define-parenscript collapse-to-focus ()
    "Collapse the selection"
  
negative.code: (define-command clear-selection ()
    "Clear the selection and unset the mark."
    (let ((mode (find-submode (current-buffer) 'visual-mode)))
      (unless (collapsed-p) (collapse-to-focus))
      (setf (mark-set mode) nil)
      (echo "Mark deactivated")))
  
  