docs: public static int xDigitToInt(int c, int accumulator)
      {
          check: {
              // Use 0..9 < A..Z < a..z
              if (c <= '9') {
                  c -= '0';
                  if (0 <= c) { break check; }
              } else if (c <= 'F') {
                  if ('A' <= c) {
                      c -= ('A' - 10);
                      break check;
                  }
              } else if (c <= 'f') {
                  if ('a' <= c) {
                      c -= ('a' - 10);
                      break check;
                  }
              }
              return -1;
          }
          return (accumulator << 4) | c;
      }