Macro tree 

macro_rules! tree {
    ($arena:expr, $root_node:expr) => { ... };
    ($arena:expr, $root_node:expr => []) => { ... };
    ($arena:expr, $key:expr => [$($value:tt)+]) => { ... };
}

Creates a tree.

Examples

let root = tree!(
    arena,
    1 => [
        2,
        3 => [4, 5],
        6 => [
            7 => [8 => []],
            9 => [10 => [11]]],
        12
    ]
);

let root = tree!(arena, 1);