drop-voicing function

Hi,

why the function doesn't work with 3 note chords?

(setf mat'(c4e4g4 e4g4c5 g4c5e5))

(tapis de drop-voicing :type '(0)) → (c4e4g4 e4g4c5 g4c5e5) ??

(tapis de voicing drop :type '(1)) → (c4e4g4 e4g4c5 g4c5e5) ??

(tapis de drop-voicing :type '(2)) → (c4e4g4 e4g4c5 g4c5e5) ??

we can drop 3-note chords.

(setf mat'(c4e4g4b4 e4g4b4c5 g4b4c5e5))

(tapis de drop-voicing :type '(0)) → (c4e4g4b4 e4g4b4c5 g4b4c5e5) ok

(tapis de drop-voicing :type '(1)) → (e3c4g4b4 g3e4b4c5 b3g4c5e5) ok

(tapis de drop-voicing :type '(2)) → (g3c4e4b4 b3e4g4c5 c4g4b4e5) ok

(tapis de drop-voicing :type '(3)) → (c3g3e4b4 e3b3g4c5 g3c4b4e5) ok

(tapis de drop-voicing :type '(4)) → (e3g3c4b4 g3b3e4c5 b3c4g4e5) ok

(tapis de drop-voicing :type '(5)) → (g2e3c4b4 b2g3e4c5 c3b3g4e5) ok

Here is a more logical classification for 4 note chords, I think it would be more logical to classify them like this