Rest staff position is displaced by voice on a shared stave, per a fixed convention, not ad hoc

When two voices share one stave (SATB grand-staff, or any stacked-voice writing), each voice's rests are displaced from the default middle-line position so they don't collide: the stem-up voice's rests sit higher, the stem-down voice's sit lower, and a voice with no forced stem direction (alone on its own stave) uses the plain centered position — consistently for every rest in that voice, not merely 'somewhere that doesn't currently collide.'

Gould, Behind Bars, ch. 'Rests', part-writing rest-displacement convention for multi-voice single-stave notation; Ross, The Art of Music Engraving, similar guidance for chorale/hymnal engraving.

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