Three or four genuinely independent beamed voices cannot be engraved on two staves — open score is structurally required

An SATB grand staff assumes at most two voices per staff and works because chorale voices are homorhythmic. Fugal writing with 3-4 independent, continuously beamed voices cannot be split 2-and-2 onto two staves without at least one of the two pairs hitting the lattice problem, because with 3-4 voices, at least one pair is forced to share a stave.

A 3- or 4-voice fugue exposition authored with `voice2`/`voice3`/`voice4` on the default (non-open-score) SATB layout, where at least one same-stave pair is classified INDEPENDENT and beamed.

Stone, Music Notation in the 20th Century.

This rule needs the notated content and the rendered result together — the score says what should happen, the page says whether it did.