code: private BigDecimal parseNumber(final NumberBuffer buffer) {
          BigDecimal sum = BigDecimal.ZERO;
          BigDecimal result = parseLargePair(buffer);
  
          if (result == null) {
              return null;
          }
  
          while (result != null) {
              sum = sum.add(result);
              result = parseLargePair(buffer);
          }
  
          return sum;
      }
docs: Parses a Japanese number
  
  @param buffer buffer to parse
  @return parsed number, or null on error or end of input