Chorale short score — four voices, two staves, stems opposed WHEN SHARED

A four-part chorale is engraved SATB on two staves (S+A on treble, T+B on bass), not as four separate staves — Gould explicitly gives this as the standard vocal short-score layout. Within each stave, the upper of the two voices sharing it takes stems UP and the lower takes stems DOWN, regardless of pitch — stem direction signals VOICE IDENTITY, not the ordinary pitch-based rule. This forcing applies ONLY when two voices actually share a stave; a voice that has a stave to itself (e.g. a 3-voice S/A+B texture, where Bass alone occupies the bass stave) correctly keeps natural pitch-based stems.

A chorale block where two voices genuinely SHARE a stave (both S+A present on treble, or both T+B on bass) but one of them renders with pitch-derived rather than forced stems — i.e., a renderer regression where `shared` was computed incorrectly, OR a chorale authored across 4 separate staves (open score) for a texture that is genuinely homophonic and doesn't need the independence open score buys.

Gould (Behind Bars, keyboard/vocal short-score chapter — fixed opposite stems keyed to voice for shared-stave pairs).

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