code: static long decTwoBytePrimaryByOneStep(long basePrimary, boolean isCompressible, int step) {
          // Extract the second byte, minus the minimum byte value,
          // minus the step, modulo the number of usable byte values, plus the minimum.
          // Reserve the PRIMARY_COMPRESSION_LOW_BYTE and high byte if necessary.
          // Assume no further underflow for the first byte.
          assert(0 < step && step <= 0x7f);
          int byte2 = ((int)(basePrimary >> 16) & 0xff) - step;
          if(isCompressible) {
              if(byte2 < 4) {
                  byte2 += 251;
                  basePrimary -= 0x1000000;
              }
          } else {
              if(byte2 < 2) {
                  byte2 += 254;
                  basePrimary -= 0x1000000;
              }
          }
          return (basePrimary & 0xff000000L) | (byte2 << 16);
      }
docs: Decrements a 2-byte primary by one range step (1..0x7f).