def add_horizontal_char(char)
      node = find_horizontal(char);
      if node != nil
        return node
      end
      node = Node.new(char, up)
      add_horizontal(node)
      return node
    end
def add_horizontal_char(char)
      node = find_horizontal(char);
      if node != nil
        return node
      end
      node = Node.new(char, up)
      add_horizontal(node)
      return node
    end
Add and return a new, or return the existing, node with the given char horizontally
 (within the left-right binary tree of this Node)