deferred class  TREE_OPERATOR[T]
   
-- This class defines the interface for user-defined operations on binary 
-- trees.  The idea is that the class TREE contains a special feature for 
-- applying operators to trees, and the user can define operators to do 
-- anything he wants.  If t is a tree and op is an operator then the command 
-- t.apply(op) applies op to t.  This means that t.apply first calls 
-- op.start, then it calls op.apply once for each node in the tree, passing 
-- the node as an argument, and then it calls op.finish.  The order in which 
-- nodes are processed is determined by the attribute `traversal', which must 
-- be initialised at creation time to one of the three values `Preorder' 
-- (visit a node before its subtrees), `Inorder' (visit a node after its left 
-- subtree but before its right) and `Postorder' (visit a node after its 
-- subtrees).  To use this facility, the user must create a new class that 
-- inherits from TREE_OPERATOR and supply an implementation for op.apply.  By 
-- default, op.start and op.finish do nothing, but they can be redefined.
   
feature {TREE}
   
   start is
      do
      end -- start
   
   finish is
      do
      end -- finish
   
   apply( n: NODE[T] ) is
      deferred
      end -- apply
   
feature {ANY}
   
   Preorder, Inorder, Postorder : INTEGER is unique
   
   traversal : INTEGER
   
invariant traversal = Preorder or traversal = Inorder or traversal = Postorder
   
end -- class TREE_OPERATOR[T]