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				% 9-13-1.tex
				\documentclass{article}
				\usepackage{amsmath}
				\begin{document}
				\[
				   a_0 + \cfrac{1}{a_1 + \cfrac{1}{a_2 +
				         \cfrac{1}{a_3 + \cfrac{1}{a_4}}}}
				\]
				
				\[ \binom{n+1}{k} \]
				
				\renewcommand{\atop}[2]{\genfrac{}{}{0pt}{}{#1}{#2}}
				\begin{equation}
				\sum_{\atop{k_0,k_1,\ldots>0}{k_0+k_1+\cdots=n}} F(k_i)
				\end{equation}
				\end{document}