RenameNotImplemented(String, String),

            FileSetError::RenameNotImplemented(old, new) => {
                write!(f, "not implemented: rename({}, {})", old, new)
            }

        Err(FileSetError::RenameNotImplemented(
            old.to_string(),
            new.to_string(),
        ))

        let mut with_slash = old.to_owned();
        with_slash.push_str("/");
        let mut files = self.files.lock().unwrap();
        for f in files.iter_mut() {
            if f.name == old {
                f.name = new.to_owned();
            } else if f.name.starts_with(&with_slash) {
                let (_, remainder) = f.name.split_at(old.len());
                let mut new_name = new.to_owned();
                new_name.push_str(remainder);
                f.name = new_name;
            }
        }
        Ok(())