Two or more grace notes before a main note should render as one beamed cluster, not separately flagged notes

When a main note is preceded by multiple grace notes, the conventional rendering beams them together under a single beam (with, in the acciaccatura case, only the first note carrying the slash) rather than giving each its own individual flag.

Gould, Behind Bars, grace notes section (multi-note grace groups are beamed); Read, Music Notation, ornaments chapter, same; Dorico and LilyPond both auto-beam multi-note grace groups by default.

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