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