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The numbers used in ac315e3e are not reproducible,
used something close instead and re-computed
reference

This follows up on 9dbf60e8

(cherry picked from commit 7ad5f7a5)

This is the algorithm by GiNaC. In theory one
could extend this to add a caching mechanism such as

    complex(kind=prec) :: cache(size(x),MPLMaxQ)

    do q=1,j
      cache(:,q) = x**q/q**m
    enddo
    do q=1,MPLMaxQ
      res = t(1)

      ! Fortran uses Column-major order, hence cache(:,q) is
      ! faster than cache(q,:).
      cache(:,q+j-1) = x**(q+j-1)/(q+j-1)**m
      t(j) = t(j) + cache(j,q)
      do k=1,j-1
        t(j-k) = t(j-k) + t(j-k+1) * cache(j-k,k+q)
      enddo

      if (mod(q,2) .eq. 1) then
        if (abs(t(1)-res).lt.MPLdel) exit
      endif
    enddo

In practice this doesn't really help because any
time saved with the cache is paid back through the
allocation and clearing of cache(:,:). Both
variations work similarly well now. If at some
point we might need MPLs with many more arguments
(size(x)), this might change.

Note that you shouldn't run this! It works but
includes loads of unnecessary casting.