pub trait RlpEncodable: RlpLength {
    fn rlp_encode<T: BufMut>(&self, encoder: &mut Encoder<T>);

    fn rlp_encode_raw<T: BufMut>(&self, encoder: &mut Encoder<T>);

    fn rlp_len(&self) -> usize {
        let raw_len = self.rlp_len_raw();
        length_of_length(raw_len) + raw_len
    }

    fn rlp_len_raw(&self) -> usize; // Important: no default!!
}

// Might not be needed.
// pub trait DynRlpEncodable: RlpLength { /* ... */ }
// pub trait RlpLength { /* rlp_len, rlp_len_raw */ }
// impl<T> DynRlpEncodable for T where T: RlpEncodable { /* ... */ }

pub trait RlpDecodable<'de> {
     fn rlp_decode(decoder: &mut Decoder<'de>) -> Result<Self>;

     fn rlp_decode_raw(decoder: &mut Decoder<'de>) -> Result<Self> {
          Self::rlp_decode(decoder)
     }
}

pub struct Encoder<T: BufMut> {
    out: T,
}

// `struct Rlp` is replaced with `Decoder`.
pub struct Decoder<'de> {
    buf: &'de [u8],
}

impl<'de> Decoder<'de> {
    // Methods moved from `Header` ...

    // Methods for creating errors that contain source information like byte position
}

pub struct Error {
    bytepos: usize,
    kind: ErrorKind,
}

pub enum ErrorKind {
    // Previous `enum Error` ...
}