anchor.diff:      (:dir (:forward :back :both :none))
       (:tailclip boolean)
       (:headclip boolean)
  -    (:arrowhead (:none :normal :inv :dot :odot :invdot :invodot :tee
  -                 :empty :invempty :open :halfopen :diamond :odiamond
  -                 :box :obox :crow))
  -    (:arrowtail (:none :normal :inv :dot :odot :invdot :invodot :tee
  -                 :empty :invempty :open :halfopen :diamond :odiamond
  -                 :box :obox :crow))
  +    (:arrowhead ,*predefined-arrow-shapes*)
  +    (:arrowtail ,*predefined-arrow-shapes*)
       (:headlabel label-text)
       (:taillabel label-text)
  +    (:ltail text)
  +    (:lhead text)
       (:labelfontsize integer)
       (:labelfontname text)
       (:labelfontcolor text)
  
positive.code:     (:style (:filled :solid :dashed :dotted :bold :invis))
      (:layer text)
      (:url text)
      (:target text)))
  
  (defparameter *edge-attributes*
    '((:minlen integer)
      (:weight integer)
      (:arrowhead ("normal" "inv" "dot" "invdot" "odot" "invodot" "none" "tee" "empty" "invempty" "diamond" "odiamond" "ediamond" "crow" "box" "obox" "open" "halfopen" "vee" "circle"))
      (:label label-text)
      (:fontsize integer)
      (:fontname text)
  
negative.code:     (:bgcolor text)
      (:url text)
      (:target text)
      (:stylesheet text)
      (:splines text)))
  
  (defparameter *node-shapes*
    '(:box :polygon :ellipse :oval
      :circle :point :egg :triangle
      :plaintext :plain :diamond :trapezium
      :parallelogram :house :pentagon :hexagon
      :septagon :octagon :doublecircle :doubleoctagon
      :tripleoctagon :invtriangle :invtrapezium :infhouse
      :Mdiamond :Msquare :Mcircle :rect
      :rectangle :square :star :none
      :underline :cylinder :note :tab
      :folder :box3d :component :promoter
      :cds :terminator :utr :primersite
      :restrictionsite :fivepoverhang :threepoverhang :noverhang
      :assembly :signature :insulator :ribosite
      :rnastab :proteasesite :proteinstab :rpromoter
      :rarrow :larrow :lpromoter :record))
  
  (defparameter *node-attributes*
    `((:height integer)
      (:width integer)
      (:fixed-size boolean)
      (:label label-text)
      (:shape ,*node-shapes*)
      (:fontsize integer)
      (:fontname text)
      (:color text)
  