Any power of 2 minus 1 is all ones: (*2 ^{ N} – 1 = 111….b*)

```
2 = 2^1. 2-1 = 1 (1b)
4 = 2^2. 4-1 = 3 (11b)
8 = 2^3. 8-1 = 7 (111b)
```

Take 8 for example. 1000 & 0111 = 0000

So that expression tests if a number is NOT a power of 2.

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