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Can someone please tell me what is wrong with the following snippet? It was generated by exporting latex from TeXmacs.

$\begin{equation}
\begin{array}{ll}
\begin{array}{l}
_p F_q \left( \begin{array}{l}
a_1, \ldots, a_p\\
b_1, \ldots, b_q
\end{array} | t \right)
\end{array} & = \sum_{k = 0 ...