This is a proposed Arial-based font that could be included in future
versions of LilyPond.
Actually, it's more a range of possibilities than a new font. Placing chord
extensions like b9 and #11 one above the other is not a new idea, and may be
easily achieved by LilyPond wizards.
For instance, the chord that appears as a C7 plus a small flat 9th above a
small sharp 11th might conceivable be rendered in LilyPond notepad code as
c:7[b9/#11]; similarly, a C9 plus a small sharp 11th above a 13th might be
rendered as c:9[#11/13]. AND this might be done with virtually ANY