use crate::extension::Signature; /// The signature of the sparse index extension, nothing more than an indicator at this time.pub const SIGNATURE: Signature = *b"sdir"; /// Serialize the sparse index extension to `out`pub fn write_to(mut out: impl std::io::Write) -> Result<(), std::io::Error> {    out.write_all(&SIGNATURE)?;    out.write_all(&0_u32.to_be_bytes())?;    Ok(())}