Macro nom::length_data

macro_rules! length_data {
    ($i:expr, $submac:ident!( $($args:tt)* )) => { ... };
    ($i:expr, $f:expr) => { ... };
}

length_data!(I -> IResult<I, nb>) => O

length_data gets a number from the first parser, than takes a subslice of the input of that size, and returns that subslice