#[cfg(test)]
mod tests {
    #[test]
    fn add_nodes() {
        let stack = vec![Node::Leaf(true), Node::Leaf(false)];
        let rhs = vec![Node::Binary {
            operator: Token::Conjunction,
            lhs: Box::new(Node::Leaf(true)),
            rhs: Box::new(Node::Leaf(false))
        }];
        assert_eq!(add_binary_node(&mut stack, Token::Conjunction), rhs);
    }
}