Module OOP {
      \\ Class is a global function (until this module end)
      Class Null {
      }
      \\ Null is a pointer to an object returned from class Null()
      Global Null->Null()
      Class Node {
      Public:
            x, Group LeftNode, Group RightNode
      Class:
            \\ after class:  anything exist one time,
            \\ not included in final object
            Module Node {
                  .LeftNode<=Null
                  .RightNode<=Null
                  Read .x
                  \\ read ? for optional values
                  Read ? .LeftNode, .RightNode
            }
      }
      \\ NodeTree return a pointer to a new Node
      Function NodeTree {
            \\ ![] pass currrent stack to Node()
            ->Node(![])
      }

      Tree=NodeTree(1, NodeTree(2,NodeTree(4, NodeTree(7)), NodeTree(5)), NodeTree(3, NodeTree(6, NodeTree(8), NodeTree(9))))

      Module preorder (T) {
            Print "preorder:    ";
            printtree(T)
            Print
            sub printtree(T as pointer)
                  If T is Null then Exit sub
                  Print T=>x;" ";
                  printtree(T=>LeftNode)
                  printtree(T=>RightNode)
            end sub
      }
      preorder Tree
      Module inorder (T) {
            Print "inorder:     ";
            printtree(T)
            Print
            sub printtree(T as pointer)
                  If T is Null then Exit sub
                  printtree(T=>LeftNode)
                  Print T=>x;" ";
                  printtree(T=>RightNode)
            end sub
      }
      inorder Tree
      Module postorder (T) {
            Print "postorder:   ";
            printtree(T)
            Print
            sub printtree(T as pointer)
                  If T is Null then Exit sub
                  printtree(T=>LeftNode)
                  printtree(T=>RightNode)
                  Print T=>x;" ";
            end sub
      }
      postorder Tree
      Module level_order (T) {
            Print "level-order: ";
            Stack New {
                  printtree(T)
                  if empty then exit
                  Read T
                  Loop
            }
            Print
            sub printtree(T as pointer)
                  If T is Null else
                        Print T=>x;" ";
                        Data T=>LeftNode, T=>RightNode
                  end if
            end sub
      }
      level_order Tree
}
OOP