Macro node_id

macro_rules! node_id {
    () => { ... };
    ($e:expr) => { ... };
    ($e:expr, $p:expr) => { ... };
    ($i:ident $e:expr) => { ... };
    ($i:ident $e:expr, $p:expr) => { ... };
}

represents a node id in dot lang Essentially it is a combination of id and port