f</code>); (2) perform the main model fitting
step by running 100 EM updates using <code>fit_poisson_nmf</code>; (3)
refine the fit by running 100 extrapolated SCD updates, again using
<code>fit_poisson_nmf</code>; and (4) recover the multinomial topic model
by calling <code>poisson2multinom</code>.</p>
<p>This two-stage fitting approach is based on our findings that the
EM algorithm initially makes rapid progress toward a solution, but
its convergence slows considerably as the iterates approach a
solution.