Courtesy accidental expected across a system/page break or after a rest interrupts an active accidental

A courtesy accidental is conventionally expected when a previously-altered pitch recurs after a system break, page turn, or a rest long enough that the reader could lose track -- even within the technically-valid same bar or into a new system's first bar.

Two occurrences of F#4 straddle a system break with no courtesy accidental, risking the reader assuming reversion to key-signature default.

Gould, system-break courtesy accidentals; Ross, explicit recommendation for orchestral parts; Read treats this as optional editorial taste rather than a rule.

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