Fractions as used by LilyPond are again stored as pairs, this time of unsigned integers. While Scheme can represent rational numbers as a native type, musically ‘2/4’ and ‘1/2’ are not the same, and we need to be able to distinguish between them. Similarly there are no negative ‘fractions’ in LilyPond’s mind. So 2/4 in LilyPond means (2 . 4) in Scheme, and #2/4 in LilyPond means 1/2 in Scheme.

Extending LilyPond v2.25.17 (development-branch).