sort collation

Dear all,

I have a <xsl:sort> which is applied to footnote numbering. The footnotes
are numbered a,b,c....aa,ab,ac...ba,bb,bc, etc

With no collation applied, the footnotes are sorted a, aa, ab, b, ba, bb,
bc, etc. I am having trouble identifying the collation to use to get them
into the order as above.

Any suggestions as to which collation to use?

Thanks
JPR