mpz_addmul_ui with ui = 1

[Paul Zimmermann]

       Marc,

> (Not directly related)
> At some point I was wondering whether the special representation of 0 
> (size==0) should be extended to represent instead small numbers that can 
> fit in an integer at most as long as _mp_d. Hard to tell.

an alternate trick is to store in the mpz_t *pointer* a value of the form
p=2k+1, where k is a small integer. Since pointers are usually even, you
can easily distinguish between small integers and GMP integers,
and you obtain the value k = p >> 1. It is also more memory-efficient since
you don't allocate the mpz_t structure. A famous computer algebra system is
using that trick together with GMP.

It might make sense to have another layer on top of mpz with that trick.

Paul Zimmermann