Function tree 

pub fn tree<S, P>(
    source: S,
    build_row: impl Fn(S::Slot) -> P + 'static,
) -> TreePiece<S>
where
    S: NodeSource + 'static,
    P: Piece,

Build a hierarchical tree from a NodeSource and a row builder: tree(branches(items_fn, key_of, parent_of), row_view) for plain data, tree(store.tree(children_of), row_view) for a day-model store.