This arena tree structure is using just a single Vec and numerical identifiers
(indices in the vector) instead of reference counted pointers. This means there
is no RefCell and mutability is handled in a way much more idiomatic to Rust
through unique (&mut) access to the arena. The tree can be sent or shared across
threads like a Vec. This enables general multiprocessing support like
parallel tree traversals.
use indextree::Arena;
// Create a new arena
let arena = &mut Arena::new();
// Add some new nodes to the arena
let a = arena.new_node(1);
let b = arena.new_node(2);
// Append a to b
assert!(a.append(b, arena).is_ok());
assert_eq!(b.ancestors(arena).into_iter().count(), 2);