\item The built-in versions have more space efficient pickles.
\item The built-in versions do not have a \method{union_update()} method.
Instead, use the \method{update()} method which is equivalent.
-\item The built-in versions do not have a \method{_repr(sort=True)} method.
+\item The built-in versions do not have a \method{_repr(sorted=True)} method.
Instead, use the built-in \function{repr()} and \function{sorted()}
functions: \code{repr(sorted(s))}.
\item The built-in version does not have a protocol for automatic conversion