mpz_t size

[Torbjorn Granlund]

You need to debug your program, inserting gmp_printf here and there
to see if you have gotten the values you expect.

Since gmp-discuss's main purpose is not to get programming help,
I'll let you spot your mistake.

A note on efficiency though: Computing mod a mersenne number
2^n-1 can be done faster than using a general division; just add
the lower n bits and the remaining higher bits.

-- 
Torbjörn