Macro monadic::wrt_mdo

macro_rules! wrt_mdo {
    (lift $nested_monad:expr                ) => { ... };
    (guard $boolean:expr ; $($rest:tt)*) => { ... };
    (_ <- $monad:expr ; $($rest:tt)* ) => { ... };
    ($v:ident <- lift $nested_monad:expr ; $($rest:tt)* ) => { ... };
    ($v:ident <- $monad:expr ; $($rest:tt)* ) => { ... };
    ($monad:expr                            ) => { ... };
}

macro for a WriterT transformer that holds a (Monad, Monoid) pair