struct LayerId

**Zero identifiers**: zero is never a valid identifier, for any of these types; each

`Create*` method rejects it. Unlike the classic API (where, e.g., [`Flatland.SetContent`]

accepts id 0 to clear a Transform's content), Flatland2 never uses a zero id to express

clearing; where clearing is possible, it is expressed by omitting an optional field

(see [`Flatland.SetTransformContent`]).

A user-defined identifier for a particular layer. See [`Flatland.CreateLayer`] and

[`Flatland.ReleaseLayer`] for more information.