Gradus has no cross-staff voice notation — a voice cannot borrow the neighboring stave for a phrase, and this gap must be documented rather than silently produce a wrong-clef or wrong-register rendering

Real engraving occasionally lets a voice's notes render temporarily on the OTHER stave of a grand staff or open score (a beam or stem crossing the gap between staves) when a keyboard line's register briefly crosses over — standard practice per Gould's chapter on cross-staff notation, used to keep a single musical line visually continuous instead of forcing awkward register-driven clef choices.

Gould, Behind Bars, ch. on cross-staff notation and beaming; Ross, The Art of Music Engraving, describes the same for keyboard writing. Not directly relevant to Gradus's SATB/fugue teaching content, but a real gap if any lesson ever wants a Bach two-part-invention-style register crossing rendered idiomatically.

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