anchor.diff:      (:constraint boolean)
       (:layer text)
       (:url text)
  -    (:target text)))
  +    (:target text)
  +    (:tooltip text)))
  
positive.code:     (:edge attribute)
      (:ranksep float)
      (:ordering (:out))
      (:rankdir ("LR" "RL" "BT"))
      (:dir ("forward" "none"))
      (:pagedir text)
      (:layout text) ;; neato, twopi, etc.
  
negative.code:     (:style (:filled :solid :dashed :dotted :bold :invis))
      (:layer text)
      (:url text)
      (:target text)
      (:tooltip text)))
  
  (defparameter *predefined-arrow-shapes*
    '(:normal :inv :dot :invdot :odot :invodot :none :tee :empty
      :invempty :diamond :odiamond :ediamond :crow :box :obox :open
      :halfopen :vee :circle)
    "See http://www.graphviz.org/doc/info/attrs.html#k:arrowType.")
  
  (defparameter *edge-attributes*
    `((:minlen integer)
      (:weight integer)
      (:label label-text)
      (:fontsize integer)
      (:fontname text)
  