/// A b-tree node storing leaves at the bottom, and with info
/// retained at each node. It is implemented with atomic reference counting
/// and copy-on-write semantics, so an immutable clone is a very cheap
/// operation, and nodes can be shared across threads. Even so, it is
/// designed to be updated in place, with efficiency similar to a mutable
/// data structure, using uniqueness of reference count to detect when
/// this operation is safe.
///
/// When the leaf is a string, this is a rope data structure (a persistent
/// rope in functional programming jargon). However, it is not restricted
/// to strings, and it is expected to be the basis for a number of data
/// structures useful for text processing.
#[derive(Clone)]
pub struct Node<N: NodeInfo>(Arc<NodeBody<N>>);