Function nested 

pub fn nested<'a, F, R>(
    input: &mut Reader<'a>,
    tag: Tag,
    decoder: F,
) -> Result<R>
where
    F: FnOnce(&mut Reader<'a>) -> Result<R>,

Return the value of the given tag and apply a decoding function to it.

extern crate derp;
extern crate untrusted;

use derp::Tag;
use untrusted::Input;

const NESTED: &'static [u8] = &[
    0x30, 0x14,                                 // seq
        0x30, 0x0c,                             // seq
            0x02, 0x04, 0x0a, 0x0b, 0x0c, 0x0d, // x
            0x02, 0x04, 0x1a, 0x1b, 0x1c, 0x1d, // y
        0x02, 0x04, 0x01, 0x02, 0x03, 0x04,     // z
];
fn main () {
    let input = Input::from(NESTED);
    let (x, y, z) = input.read_all(derp::Error::Read, |input| {
        derp::nested(input, Tag::Sequence, |input| {
            let (x, y) = derp::nested(input, Tag::Sequence, |input| {
                let x = derp::positive_integer(input)?;
                let y = derp::positive_integer(input)?;
                Ok((x, y))
            })?;
            let z = derp::positive_integer(input)?;
            Ok((x, y, z))
        })
    }).unwrap();

    assert_eq!(x.as_slice_less_safe(), &[0x0a, 0x0b, 0x0c, 0x0d]);
    assert_eq!(y.as_slice_less_safe(), &[0x1a, 0x1b, 0x1c, 0x1d]);
    assert_eq!(z.as_slice_less_safe(), &[0x01, 0x02, 0x03, 0x04]);
}