Macro zero_tree 

macro_rules! zero_tree {
    ($V:ty) => { ... };
    ($V:ty, fixed) => { ... };
    ($V:ty, $H:ty) => { ... };
    ($V:ty, $H:ty, fixed) => { ... };
}

Expands to ZeroTree<<V as CollectionMeta>::SelfId, { size_of::<V>() }, V [, H, D]>.