%%
%% \expandstr{width}{string}
%%
\def\expandstr#1#2{\leavevmode{\def\@tempa{#2}%
  \ifx\@tempa\empty\hbox to#1{\hss}%
  \else\relax\hbox to#1\bgroup\@expandstr#2\end@expandstr\fi}}
\def\@expandstr#1#2\end@expandstr{\def\@tempa{#2}#1%
  \ifx\@tempa\empty\let\next\end@expandstr
  \else\hfil\def\next{\@expandstr#2\end@expandstr}\fi
  \next}
\let\end@expandstr\egroup

\endinput