Seems lilypond changed its way of specifying fractional tuplet-numbers after v.2.8. Heres a diff to fomus' lilypond-backend to suite:

cd /div/notam02/site/localdomain/cm-sys/fomus/ diff -c /div/notam02/site/localdomain/cm-sys/fomus/backend_ly.lisp~ /div/notam02/site/localdomain/cm-sys/fomus/backend_ly.lisp *** /div/notam02/site/localdomain/cm-sys/fomus/backend_ly.lisp~ 2007-03-26 14:40:05.000000000 +0200 --- /div/notam02/site/localdomain/cm-sys/fomus/backend_ly.lisp 2007-04-10 12:44:48.000000000 +0200 *************** *** 289,295 **** (loop for s from 1 to ns do (format f " \change Staff = ~A \override Staff.TimeSignature #'style = #'()~%" (code-char (+ 64 s)))) (format f " \override Staff.TimeSignature #'style = #'()~%"))) ! (when (eq *tuplet-style* :ratio) (format f " \set tupletNumberFormatFunction = #fraction-tuplet-formatter~%")) (format f " \autoBeamOff~%") (if *acc-throughout-meas* (format f " #(set-accidental-style 'default)~%") --- 289,297 ---- (loop for s from 1 to ns do (format f " \change Staff = ~A \override Staff.TimeSignature #'style = #'()~%" (code-char (+ 64 s)))) (format f " \override Staff.TimeSignature #'style = #'()~%"))) ! (when (eq *tuplet-style* :ratio) (format f " \override TupletNumber #'text = #tuplet-number::calc-fraction-text~%")) ! ! (format f " \autoBeamOff~%") (if *acc-throughout-meas* (format f " #(set-accidental-style 'default)~%")

Diff finished at Tue Apr 10 12:45:04