Courtesy natural expected when a diatonic-looking note contradicts the key signature after recent chromatic activity

After recent chromatic alteration of a letter, a nearby return to that letter's plain key-signature spelling is sometimes given a courtesy mark to reassure the reader, though not technically required.

A passage tonicizes vi with a raised leading tone for two bars, then returns to diatonic material with no reminder.

Gould and Ross both note this as good practice but not inviolable.

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