- Testing requirements after changes:

  - Test all functions return either native or bigints.  Functions that
    return raw MPU::GMP results will return strings, which isn't right.

  - Valgrind, coverage

  - compile with -O2 -g -Wall -Wextra -Wdeclaration-after-statement -fsigned-char



- Add test to check maxbits in compiled library vs. Perl

- Figure out documentation solution for PP.pm

- Is the current PP.pm setup the way we want to do it?

- Move .c / .h files into separate directory.
  version does it in a painful way.  Something simpler to be had?

- finish test suite for bignum.  Work on making it faster.

- Test all routines for numbers on word-size boundary, or ranges that cross.

- More testing on 32-bit machines.

- An assembler version of mulmod for i386 would be _really_ helpful for
  all the non-x86-64 Intel machines.

- More efficient Mertens.  The current version has poor growth.

- It may be possible to have a more efficient ranged totient.  We're using
  the sieve up to n/2, which is better than most people seem to use, but I'm
  not completely convinced we can't do better.

- Big features:
   - QS factoring

- segment sieve should itself use a segment for its primes.
  Today we'd need sqrt(2^64) max = 140MB.  Segmenting would yield under 1MB.

- Figure out a way to make the internal FOR_EACH_PRIME macros use a segmented
  sieve.

- Rewrite 23-primality-proofs.t for new format (keep some of the old tests?).

- Use Montgomery routines in more places: Factoring.

- Put euler_phi and moebius directly in XS.
    (1) optional second argument.  Easily handled, and not hard to do in
        generic sub call.
    (2) generic sub returns an array.  This is the sticking point.

- Factoring in PP code is really wasteful -- we're calling _isprime7 before
  we've done enough trial division, and later we're calling it on known
  composites.  Note how the XS code splits the factor code into the public
  API (small factors, isprime, then call main code) and main code (just the
  algorithm).  The PP code isn't doing that, which means we're doing lots of
  extra primality checks, which aren't cheap in PP.

- Consider using Test::Number::Delta for many tests

- More tweaking of LMO prime count.
    - Consider using UV size for sieve
    - is there any way to see if we could safely use asm popcnt?
    - OpenMP.  The step 7 inner loop is available.
    - Convert to 32-bit+GMP to support large inputs, add to MPU::GMP.

- Add Inverse Li and Legendre Phi to API?
