\begin{psyntax}{External directive}{externaldirective}
\synt{external\ directive}
\lit*{external}
\begin{stack}\\
  \synt{string\ constant}
  \begin{stack}\\
    \lit*{name} \synt{string\ constant}\\
    \lit*{index} \synt{integer\ constant}
  \end{stack}
\end{stack}
\end{psyntax}