Macro monadic::rdrdo

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