\cdbproperty{KeepHistory}{}

This is a global property. When set to false, only the last version of
each expression is kept in memory, thereby reducing memory usage for
very large expressions. The default is ``true''. When set to
``false'', \subscommand{pop} is no longer available to return to
previous forms of the expression:
\begin{screen}{1,2,3,4}
::KeepHistory(false).
(a+b)*(c+d);
@distribute!(%);
@pop(%);
@pop: not applicable.
\end{screen}
In general this is only useful to conserve memory when extremely long
expressions are manipulated.

\cdbseealgo{pop}
