>>>>> "David" == David J Birnbaum <djbpitt+xml@xxxxxxxx> writes:

    David> What I'm groping for, then, is an elegant rule-based
    David> function that lets me write a small number of rules by
    David> defining classes of letters to which they apply, something
    David> like "after 'S', 'Z', 'C', 'St', and Zd', 'y' is replaced
    David> by 'E'." As I mention above, these rules apply only at the
    David> boundary of stem plus ending; "S" can be followed by "y"
    David> elsewhere in a word. Since I've encoded my stems with
    David> trailing hyphens, I can easily distinguish "Sy" (which
    David> should be left alone) from "S-y" (which should be replaced
    David> by "SE").

Have you checked to see if the classes can be deduced from the Unicode
database? In Unicode 5.1.0, this is now available as an XML dataset.
-- 
Colin Adams
Preston Lancashire