Function on_own_nodes 

pub fn on_own_nodes(
    model: &mut Model,
    root: &Shape,
    parts: &[Shape],
    run: impl FnOnce(&mut Model, &Shape, &[Shape]) -> OgeomResult<Built>,
) -> OgeomResult<Built>

run on root and the shapes parts below it, once every node below root is the root’s alone: what other shapes hold as well is copied first, root itself included where it is held whole (Model::unshare_each), and each part is carried to its copy. The history runs from the shapes as given.

What an operation that returns what it makes of a shape, and edits below it on the way, runs inside.

Errors

As Model::unshare, and as run.