  We will tackle a conjecture of S. Seo and A. J. Yee, which says that the
series expansion of $1/(q,-q^3;q^4)_\infty$ has nonnegative coefficients. Our
approach relies on an approximation of the generally nonmodular infinite
product $1/(q^a;q^M)_\infty$, where $M$ is a positive integer and $a$ is any of
$1,2,\ldots,M$.
