Public-API lyrics carry no verse/voice distinction — two voices with different text on a shared stave will collide into the same lyric line

When two voices sharing a stave each carry their own lyric text (e.g. a hymn's melody and a descant, or two chorale parts each texted independently), MusicXML distinguishes them with a `<lyric number="N">` verse attribute, which Verovio (and every other MusicXML reader) uses to stack multiple simultaneous lyric lines vertically below the stave, one row per verse number, so each voice's text stays legible and distinct from the other's.

MusicXML specification's `<lyric number>` attribute is the standard mechanism for this; Gould, Behind Bars, and Read, Music Notation, both describe multi-verse/multi-voice underlay as requiring vertically stacked, distinguishable text rows.

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