def parse_teban
      s0 = @current_pos
      s1 = match_str("+")
      if s1 != :failed
        @reported_pos = s0
        s1 = 0
      end
      s0 = s1
      if s0 == :failed
        s0 = @current_pos
        s1 = match_str("-")
        if s1 != :failed
          @reported_pos = s0
          s1 = 1
        end
        s0 = s1
      end
      s0
    end
def parse_teban
      s0 = @current_pos
      s1 = match_str("+")
      if s1 != :failed
        @reported_pos = s0
        s1 = 0
      end
      s0 = s1
      if s0 == :failed
        s0 = @current_pos
        s1 = match_str("-")
        if s1 != :failed
          @reported_pos = s0
          s1 = 1
        end
        s0 = s1
      end
      s0
    end
teban : "+" | "-"