mpihelp_divrem
q_limb = mpihelp_divrem(qp, 0, rp, rsize, dp, dsize);
mpi_limb_t mpihelp_divrem(mpi_ptr_t qp, mpi_size_t qextra_limbs,
mpihelp_divrem(xp + msize, 0, xp, xsize,
mpihelp_divrem(xp + msize, 0,
mpihelp_divrem(rp + msize, 0, rp, rsize, mp, msize);
mpihelp_divrem(bp + msize, 0, bp, bsize, mp, msize);