\version "2.18.0"

% The property @code{chordNameExceptions} can be used to store a list of
% special notations for specific chords.

% modify maj9 and 6(add9)
% Exception music is chords with markups

chExceptionMusic = {
  <c e g b d'>1-\markup { \super "maj9" }
  <c e g a d'>1-\markup { \super "6(add9)" }
}

% Convert music to list and prepend to existing exceptions.
chExceptions = #( append
                  ( sequential-music-to-chord-exceptions chExceptionMusic #t)
                  ignatzekExceptions)

theMusic = \chordmode {
  g1:maj9 g1:6.9
  \set chordNameExceptions = #chExceptions
  g1:maj9 g1:6.9
}

<< \context ChordNames \theMusic
   \context Voice \theMusic
>>