What is a nail?

[Paul Zimmermann]

> What is a nail? Is it a bit? Nail is mentioned in mpz_import function.
> Perhaps it is relevant only for some kind of Cray system? It's unclear
> to me exactly what nail means and what it might be used for.

yes a nail is a guard bit kept to store some intermediate carries.
For example with nail=4 on a 64-bit processor you will use only 60
bits per word.

Paul Zimmermann