Round A4 up to a multiple of 4 total pages. I can't easily get the number of actual pages processed, so rely on telling the (rather hacky, and surprising - I have to divide to a separate counter to force an integer result) macro the number of pages used before the current numbering started.

RedirectMatch permanent "/Session(.*)/MazurkaAvignon\.html$" "/tunes$1/MazurkaAuvergne/index.html"
RedirectMatch permanent "/Session(.*)/([A-Z].*)\.html$" "/tunes$1/$2/index.html"
RedirectMatch permanent "/Session(.*)/learner-([A-Z].*)\.html$" "/tunes$1/$2/learner.html"