anchor.diff:  
   (defun parse-prompt (parser &optional msg &key default 
   					    (required-p t) 
  -					    if-wrong-input
  +					    validator
  +					    if-invalid
   					    (color *prompt-color*)
   					    (error-color *prompt-error-color*))
     (loop do
  
positive.code: 	   (prompt msg :default default
  		   :required-p required-p
  		   :validator (clavier:valid-url)
  		   :if-invalid (or if-wrong-input
  				   (lambda (&optional value)
  				     (msg "Invalid url" :color error-color)))
  		   :color color
  
negative.code: 	   (return-from prompt-url
  	     (prompt-url msg :default default
  			 :required-p required-p
  			 :if-invalid if-invalid
  			 :color color
  			 :error-color error-color
  			 :probe probe
  