code: public static long incTwoBytePrimaryByOffset(long basePrimary, boolean isCompressible,
                                                int offset) {
          // Extract the second byte, minus the minimum byte value,
          // plus the offset, modulo the number of usable byte values, plus the minimum.
          // Reserve the PRIMARY_COMPRESSION_LOW_BYTE and high byte if necessary.
          long primary;
          if(isCompressible) {
              offset += ((int)(basePrimary >> 16) & 0xff) - 4;
              primary = ((offset % 251) + 4) << 16;
              offset /= 251;
          } else {
              offset += ((int)(basePrimary >> 16) & 0xff) - 2;
              primary = ((offset % 254) + 2) << 16;
              offset /= 254;
          }
          // First byte, assume no further overflow.
          return primary | ((basePrimary & 0xff000000L) + ((long)offset << 24));
      }
docs: Increments a 2-byte primary by a code point offset.