ly:pitch and string

[Simon Albrecht]

Hello,

consider the following setup:
%%%%%%%%%%%%%%
\version "2.19.12"

newTonic = d

newTonicString = "d"

\bookOutputSuffix #(string-append "in-" newTonicString)

\score { \transpose c \newTonic { c' } }

%%%%%%%%%%%%%%

For easy handling of different transpositions the ‘destination pitch’ is 
stored in a variable. It is of type ly:pitch?.
At the same time, I want to flag the output file with the key to which 
the music has been transposed, and for this I need the destination pitch 
notename as a string.
I’d like to avoid having to synchronise newTonic and newTonicString 
manually, but I’ve been unable to find a convenient way to convert one 
into the other. Can anybody help?
I thought of #(ly:parser-include-string ), but it returns an entire 
sequential music expression instead of the pitch alone, plus it’s 
difficult to wrap. And the other direction (compute newTonicString from 
newTonic) seems to be even more difficult to achieve.

TIA, Simon