[xsl] XSL FO TOC for an indexing document

["Mark Wilson" <mark@xxxxxxxxxxxx>]

All of the XSL FO TOC generating examples I have seen are keyed to the 
structural parts of a book (a chapter element, etc) or some similar object. 
I have constructed an index to a journal using XSL-FO and would like to have 
a TOC that indicates the initial use of each letter of the alphabet, or some 
useful refinement thereof. For a start, how would I identify the position in 
the index where a letter was used for the first time. If it would help, here 
is a draft copy of the document: 
http://www.knihtisk.org/library/specialist/headings.pdf.

The initial occurrence of a letter , as well as every other occurrence, will 
appear in the XML element <Heading>. For the letter 'A', this is  the first 
entry in the above document <Heading>Abbeys and Monasteries</Heading>. Since 
every entry appears in the same element type, how do I distinguish 
'interesting' <Heading> elements (those with an initial occurrence of a 
letter) from their less interesting cousins (those with later occurrences)?

Any suggestions greatly appreciated.
Mark