Provides commands \alphalph and \AlphAlph. They are like
\number but the expansion consists of lowercase and uppercase
letters respectively (1 to a, 26 to z, 27 to aa, 52 to zz, 53
to ba, 702 to zz, 703 to aaa, etc.). Can be used as a
replacement for LaTeX's \@alph and \@Alph macros.
