anchor.diff:  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 ()
  +(define-parenscript collapsed-p ()
  +  "Return T if mark's start and end are the same value, nil otherwise."
  +  (defun collapsed-p ()
       (let ((sel (ps:chain window (get-selection))))
         (ps:@ sel is-collapsed)))
  -  (is-collapsed))
  +  (collapsed-p))
   
   (define-parenscript collapse-to-focus ()
     "Collapse the selection"
  
positive.code: (define-command toggle-mark ()
    "Toggle the mark."
    (let ((mode (find-submode (current-buffer) 'visual-mode)))
      (if (string= (is-collapsed) "true")
          (progn
            (setf (mark-set mode) (not (mark-set mode)))
            (if (mark-set mode)
  
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")))
  
  