mod_2

[Paul Zimmermann]

>   What are the current users of the fraction feature?
> 
> Perhaps MPFR.  Our function mpn/generic/get_str.c passes 1 for the
> fraction argument.

in mpfr_div we currently use mpn_divrem (which interface is exactly what we
need in MPFR, whereas mpn_tdiv_qr would oblige us to allocate an extra limb,
which in most cases will be 0, in contrary with mpfr_div we directly store the
quotient in the result destination).

If mpn_divrem would use that improved div_2 function, MPFR will benefit from
it.

Paul