Breath marks and caesura have zero representation on any of the three Gradus authoring surfaces

A breath mark (comma-shaped tick above the staff) indicates a brief lift/breath, with the borrowed time typically taken from the preceding note; a caesura ("railroad tracks", //) indicates a brief but essentially complete interruption of sound across the ensemble, more definitive than a breath mark. Both are standard phrasing devices distinct from a rest, a fermata, or a tie.

Gould, Behind Bars, ch. Breath Marks and Caesuras — treats both as standard, frequently-needed phrasing marks distinct from rests/fermatas; Read, Music Notation, gives the same distinction (breath mark = brief lift with implied borrowed time.

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