Hello,

I am trying to used crc64 function in crc_common.hpp to compute crc64. However is is nor clear to me what should be the `const poly64_t constants[] parameter. According to the function description: `

```
/**
* Computes a CRC64 in big- or little-endian mode using the specified shifts
* and polynomials. This can be used for smaller polynomials by shifting
* them to a degree 64 polynomial.
*
* @tparam BarretShift the shift used when computing @c ls1_divp.
* @param[in] size number of bytes of the given buffer
* @param[in] input points to the input byte sequence
* @param[out] crc24 the computed CRC on 24 bits
* @param[in] constants the constants specific to each polynomial:
constants[0] = padding
constants[1] = (1<<128) / P - (1<<64)
constants[2:11] = [ (1<<(64*k)) mod P,
for k in [1,1,2,3,4,5,6,7,8,9] ]
*/
```

What "constants[1] = (1<<128) / P - (1<<64)" stands for? The result doesnt seems to be a 64th bit polynomial.

Thanks