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If the data which the player relies on changes, the player must \texttt{setup\_pieces()} before it \texttt{pass()}es otherwise it may crash.

I think there are two approaches to this:

\begin{enumerate}

\item Playlist signals that it is going to change; player and
butler respond by suspending themselves (so they don't touch the `bad'
playlist), playlist signals when the change has been made and
\texttt{setup\_pieces()} has happened.  Player and butler resume.

\item There is some RCU system so that changes to the playlist happen in a copy
which is made into the `current' version at a convenient time to the
player/butler.  This seems a bit tricky because the player's output also
depends on the state of \texttt{Film}, so it seems that the copy would need to
include the film's state (and possibly other bits).

\end{enumerate}

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