Macro btree 

macro_rules! btree {
    ($val:expr, $l:expr, $r:expr) => { ... };
    ($val:expr, , $r:expr) => { ... };
    ($val:expr, $l:expr,) => { ... };
    ($val:expr) => { ... };
}

Short-hand for constructing Trees. btree! takes 3 arguments, the first being the value of the root node, and the other two being child nodes. The last two arguments are optional.

use gemla::tree::*;
use gemla::btree;

// A tree with two child nodes.
let t = btree!(1, btree!(2), btree!(3));
assert_eq!(t,
    Tree::new(1,
        Some(Box::new(Tree::new(2, None, None))),
        Some(Box::new(Tree::new(3, None, None)))));

// A tree with only a left node.
let t_left = btree!(1, btree!(2),);
assert_eq!(t_left,
    Tree::new(1,
        Some(Box::new(Tree::new(2, None, None))),
        None));

// A tree with only a right node.
let t_right = btree!(1, , btree!(3));
assert_eq!(t_right,
    Tree::new(1,
        None,
        Some(Box::new(Tree::new(3, None, None)))));

// A tree with no child nodes.
let t_single = btree!(1);
assert_eq!(t_single,
    Tree::new(1,
        None,
        None));