Commands to assist the reuse of boxes (set up by \sbox or
by the lrbox environment); the \settobox...
commands behave similarly to the \settowidth (etc.)
commands. For example:
\newsavebox{\mybox}
\newlength{\mylength}
\sbox{\mybox}{Hello World}
\settoboxwidth{\mylength}{\mybox}
The package is part of the oberdiek bundle.