Macro id 

macro_rules! id {
    (unchecked $id1:ident : $id2:ident) => { ... };
    (unchecked $id1:expr, $id2:expr) => { ... };
    (unchecked $id:ident) => { ... };
    (unchecked $id:expr) => { ... };
    ($id1:ident, $id2:ident) => { ... };
    ($id1:expr, $id2:expr) => { ... };
    ($id:ident) => { ... };
    ($id:expr) => { ... };
}