def parse_xy_piece
      s0 = @current_pos
      s1 = parse_xy
      if s1 != :failed
        s2 = parse_piece
        if s2 != :failed
          @reported_pos = s0
          s0 = { "xy" => s1, "piece" => s2 }
        else
          @current_pos = s0
          s0 = :failed
        end
      else
        @current_pos = s0
        s0 = :failed
      end
      s0
    end
def parse_xy_piece
      s0 = @current_pos
      s1 = parse_xy
      if s1 != :failed
        s2 = parse_piece
        if s2 != :failed
          @reported_pos = s0
          s0 = { "xy" => s1, "piece" => s2 }
        else
          @current_pos = s0
          s0 = :failed
        end
      else
        @current_pos = s0
        s0 = :failed
      end
      s0
    end
xypiece : xy piece