def all_partials(str = "")
      list = []      
      @down.walk(str) { |str| list << str } unless @down.nil?
      list
    end
def all_partials(str = "")
      list = []      
      @down.walk(str) { |str| list << str } unless @down.nil?
      list
    end
Return an Array of Strings of all possible partial string from this node.
 Optionally, use the given prefix String str.