Hookpad considers sixth chords to be minor seventh chords in first inversion. For example, E6 in the key of E major would be vi6/5 in hookpad. Also, the only way to get sixth chords in hookpad is to put a minor seventh chord in its first inversion. I want there to be an option to add sixths to major chords. Also the roman numerals to be changed to Iadd6, IVadd6, etc.
add13s are automatically displayed as add6 when the seventh is absent.
As a bonus, add9 and add13 together imply 6/9 chords when the seventh is absent.