A pickup (anacrusis) measure contains only the pickup note(s) — never rests padding it to full-bar length

An anacrusis is a genuinely short first bar; the 'missing' beats are represented by the bar simply being short, not by rests filling the gap. The shortfall is compensated by shortening the piece's final bar by the same amount.

A 4/4 piece with a one-beat pickup note authored as a full 4-beat first bar: [rest q, rest q, rest q, pickup-note q] — padding the silence with three explicit quarter rests before the pickup, rather than a genuinely one-beat-long opening bar. Confirmed Gradus has no dedicated pickup construct today: the existing `countIn` prop is documented (lines 369-378) as a PLAYBACK/metronome count-in, not a notated pickup — it adds click beats before playback starts and is folded into the dev-check's beat-total arithmetic (line 684, `pickupBeats`), but the note array itself still represents a full bar's worth of content unless an author manually shortens it with padding rests.

Gould, Behind Bars, ch. 'Bar lines and repeat marks', anacrusis/pickup-measure convention; Read, Music Notation, same; Ross, The Art of Music Engraving, agrees, no notable disagreement among the four authorities.

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