Replacing a certain character sequence in lyrics

[Simon Albrecht]

Hello everybody,

I have a favour to ask from any LilyPond-Guile crack on the list, 
because I couldn’t quite figure it out myself: (technical requirement 
here, rationale in PS.)

For a scholarly edition project, I’d like to have a global boolean 
variable and a toplevel music function that goes through all lyric 
syllables and replaces (for example) two apostrophes "’’" with either a 
single apostrophe "’" or nothing, depending on the value of that global 
switch variable. How could such a music function be done?

Best, Simon

PS. The reason behind this is: Haydn’s „Jahreszeiten“ use a lot of 
apostrophes in the original German text; some of them seem weird to the 
modern eye and don’t help understanding, while others are perfectly 
normal. I would like to be able to easily switch between modernising 
spelling in that regard or just leaving it as is, and I figured the way 
to go would be coding the ‘weird’ ones as double apostrophes and having 
a toplevel-music-function deal with them.