Function select_smallest_subtree 

pub fn select_smallest_subtree<T, M, R>(
    _: &T,
    rs: &mut dyn Iterator<Item = (&R, Update<T, M>)>,
) -> Option<Update<T, M>>
where
    T: Uniplate,
    R: Rule<T, M>,

Selects the Update which results in the smallest subtree.

Subtree size is determined by maximum depth. Among trees with the same depth, the first in the iterator order is selected.

See the module-level documentation for more information.