Adding parallel thirds to the notes of a melody

[Gianmaria Lari]

The following "test" function adds parallel thirds to the notes of a melody:

\version "2.21.0"

scale = {c d e f g a b}

test  = 

#(define-music-function (music) (ly:music?) 

   #{ 

     \partCombine $music \modalTranspose c e \scale $music

   #})

\test {\voiceOne a b c'}

I don't have lilypond programming experience and I would like to know:

- if there are hidden mistakes

- if it is better to write the function in a different way

- how I could avoid to write \voiceOne in \test {\voiceOne a b c'}

- how I can encapsulate scale, inside the "test" function

Thank you, g.