Macro flat_combine_len 

macro_rules! flat_combine_len {
    (<$t:ty>) => { ... };
    (<$t:ty> <$e:expr, $le:ident, $re:ident> $(, $($tt:tt)*)?) => { ... };
    (<$t:ty> $e:expr $(, $($tt:tt)*)?) => { ... };
}