A slash through the grace note's stem distinguishes acciaccatura from appoggiatura — Gradus's schema cannot express either intent

A single small grace note with a diagonal slash through its stem means acciaccatura — played as fast as possible, before the beat, 'crushed' against the main note. The same grace note WITHOUT the slash means appoggiatura — a more deliberate, often on-the-beat, leaning note.

Gould, Behind Bars, grace notes section (slash = acciaccatura, no slash = appoggiatura); Read, Music Notation, ornaments chapter, same; LilyPond (`\acciaccatura` vs `\appoggiatura`) and Dorico's grace-note popover model this as two distinct authorable note types.

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.