nom::length_value!

macro_rules! length_value {
    ($i:expr, $f:expr, $g:expr) => { ... };
    ($i:expr, $f:expr, $g:expr, $length:expr) => { ... };
}

length_value!(I -> IResult<I, nb>, I -> IResult<I,O>) => I -> IResult<I, Vec<O>> gets a number from the first parser, then applies the second parser that many times