code: private long toLong() {
          assert (intLen <= 2) : "this MutableBigInteger exceeds the range of long";
          if (intLen == 0)
              return 0;
          long d = value[offset] & LONG_MASK;
          return (intLen == 2) ? d << 32 | (value[offset + 1] & LONG_MASK) : d;
      }
docs: Convert this MutableBigInteger to a long value. The caller has to make
  sure this MutableBigInteger can be fit into long.