for _nok()
* Function: int mpz_fits_ulong_p (mpz_t op)
  Function: void mpz_bin_ui (mpz_t rop, mpz_t n, unsigned long int k)

* _lsft() and _rsft() could probably be a bit more optimized
* move _log_int() to XS
* _digit() should be in XS
* _zeros() could be more optimized (divide by 1e5, check for == 0?)
* replace all malloc() with New() and all free() with Savefree()?

Please send me test-reports, your experiences with this and your ideas - I love
to hear about my work!

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