Cancelling a double sharp/flat to a single accidental: modern convention vs. natural-plus-accidental

Cancelling a double sharp/flat down to a single sharp/flat on the same letter, modern convention uses JUST the single accidental, no natural.

F## earlier in the bar cancels to plain F# with a natural-then-sharp pair instead of a single sharp glyph.

Gould, explicit modern-convention statement. OLDER PRACTICE (some historically-inclusive treatments) writes natural-plus-accidental. Dorico and LilyPond default to the modern single-accidental convention.

AUTHORITIES DISAGREE; RECOMMEND Gould's modern convention for Gradus since its curriculum uses modern rendering throughout, not facsimile historical editions.

Our own judgement, not a sourced convention — shown separately so the two are never confused.

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