Generating pitches depending on key signature

[Lukas-Fabian Moser]

Folks,

Assume I want a function that adds a second note a fifth a bove a given 
pitch (of course this is but an abstraction from a more complicated 
project).

The difficulty is that this should behave accoding to the key signature 
in place. Hence, the fifth should not be always perfect but instead be 
whatever occurs in the active key four note steps above the given pitch. 
(Quite as in figured bass accompaniment.) Hence:

\version "2.19.80"

addFifth = #(define-music-function (pitch dur) (ly:pitch? ly:duration?)
              (let*
               ((delta (+ 4 (ly:pitch-steps pitch)))
                (new-pitch (ly:make-pitch 0 delta 0))
                (pitches (list pitch new-pitch) )
                (chordNotes (map (lambda (p) (make-music
                                              'NoteEvent
                                              'duration dur
                                              'pitch p))
                              pitches))
                )
               #{
                 #(make-music 'EventChord 'elements chordNotes)
               #}
               )
              )

{
  \addFifth c' 1  % should yield <c g> -> ok
  \addFifth cis' 1    % should yield <cis g> -> ok
  \key a \major
  \addFifth c' 1   % should yield <c gis> -> NOT OK
  \addFifth cis' 1  % should yield <cis gis> -> NOT OK
}

I expect that this amounts to taking the alteration of my new-pitch from 
the alteration-alist that is "active". But from what I can gather from 
old postings on -user, this information seems not to be available at the 
time a music function is called (correct?). What would be the way to go 
for this?


Best
Lukas