Laissez-vibrer (let-ring) ties have no representation in either schema

An l.v. tie (harp, piano pedal points, guitar, some percussion writing) indicates the note should ring without a following note to tie to — a tie 'into nothing,' conventionally a short curved stub after the final note with no destination notehead.

Gradus's model has no way to express this at all — `tie: true`/`tiedToNext: true` both REQUIRE a following note of identical pitch; there is no `lv: true` or equivalent flag. Any lesson wanting to teach or notate an l.v. tie (e.g. a piano pedal-point exercise) cannot do so.

Gould ('Laissez vibrer') documents this convention as distinct from an ordinary tie.

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