Macro monadic::mdo

macro_rules! mdo {
    (let $v:ident = $e:expr ; $($rest:tt)*) => { ... };
    (guard $boolean:expr ; $($rest:tt)*) => { ... };
    (_ <- $monad:expr ; $($rest:tt)* ) => { ... };
    ($v:ident <- $monad:expr ; $($rest:tt)* ) => { ... };
    (&$v:ident <- $monad:expr ; $($rest:tt)* ) => { ... };
    ($monad:expr                            ) => { ... };
}