declare
Scale=30
proc {DepthFirst Tree Level LeftLim ?RootX ?RightLim}
   case Tree
   of tree(x:X y:Y left:leaf right:leaf ...) then
      X=RootX=RightLim=LeftLim
      Y=Scale*Level
   [] tree(x:X y:Y left:L right:leaf ...) then
      X=RootX
      Y=Scale*Level
      {DepthFirst L Level+1 LeftLim RootX RightLim}
   [] tree(x:X y:Y left:leaf right:R ...) then
      X=RootX
      Y=Scale*Level
      {DepthFirst R Level+1 LeftLim RootX RightLim}
   [] tree(x:X y:Y left:L right:R ...) then
         LRootX LRightLim RRootX RLeftLim
      in
         Y=Scale*Level
         {DepthFirst L Level+1 LeftLim LRootX LRightLim}
         RLeftLim=LRightLim+Scale
         {DepthFirst R Level+1 RLeftLim RRootX RightLim}
         X=RootX=(LRootX+RRootX) div 2
   end
end