Function undelete 

pub async fn undelete<S>(
    store: &S,
    key: &RecordKey,
    now_ms: i64,
    author: Option<&Identity>,
) -> Result<Revision>
where
    S: Store + BlobStore + ?Sized,

Restore the record at key from its tombstone, returning the revision written. author, when given, is stamped as the restorer; None keeps the tombstone’s author.

The body, kind and meta come back; links do not, because a tombstone never kept them.

Refuses, without writing anything, when there is no tombstone, the record is live, the tombstone’s body was not retained, a blob the body names is missing, or the key changed while this ran.