Tie curvature (StaveTie), unlike slurs, IS built per-voice today — but relies entirely on VexFlow's own default direction, never confirmed against a real shared-stave two-tie case

Like slurs, a tie's curvature on a shared stave should curve away from the stem side so it doesn't collide with the opposite voice's stem or notehead. Unlike the slur code (explicitly voice-1-only), Gradus's tie construction runs for EVERY voice, so this is a live path today, not a latent one.

Two voices sharing a stave, both with ties active at the same passage, where one voice's tie curve collides with the other voice's notehead or stem because neither tie's curvature was explicitly set relative to its own voice's stem role.

Gould, Behind Bars, and Read, Music Notation, ch. on ties — same directional-clearance logic as slurs.

This rule is about where marks land on the page, so it can only be judged after engraving. Collision and spacing faults live here.