Two simultaneous articulations on one note (e.g. accent + staccato, a portato/marcato-staccato combination) must combine or stack per convention — currently unrepresentable in Gradus, which is itself the correct state

Combined articulations (accent+staccato, tenuto+staccato as portato) are engraved either as a single combined glyph or as two small marks stacked tightly on the same side, never as two full-size marks independently spaced per vs-articulation-notehead-clearance-gap's normal band.

A future Gradus content change adds an `articulations: string[]` field (plural) to MelodyNote and authors two marks on one note without any combined-glyph or tight-stack logic, producing two full-margin marks that read as unrelated rather than as one combined instruction.

Gould, Behind Bars, ch. 'Articulation' — combined-marking conventions; Read, Music Notation covers portato explicitly.

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