def walk(str="", &block)
      @down.walk(str+char, &block) if @down != nil
      @left.walk(str, &block) if @left != nil
      yield str+@char, @value if @value != nil
      @right.walk(str, &block) if @right != nil
    end
def walk(str="", &block)
      @down.walk(str+char, &block) if @down != nil
      @left.walk(str, &block) if @left != nil
      yield str+@char, @value if @value != nil
      @right.walk(str, &block) if @right != nil
    end
Walk the tree from this node, yielding all strings and values where the
 value is not nil. Optionally, use the given prefix String str.