- * If no byte of x is in {0, 0x80}, then the highest bit of each byte
- * of x_2 is the same as of x. Hence x_3 has all these highest bits
- * cleared. If a == 0, then we are sure there is no zero byte in x.
+ * If all bytes of x are nonzero, then the highest bit of each byte of
+ * x_2 is lower or equal to the corresponding bit of x. Hence x_3 has
+ * all these highest bits cleared (the target bit is set iff the source
+ * bit has changed from 0 to 1). If a == 0, then we are sure there is
+ * no zero byte in x.