- Tags are hex-encoded by replacing every byte not matching
- the regex **[A-Za-z0-9@=.,\_+-]** with **%nn** where nn is
- the two digit hex encoding. The message ID is a valid
- Xapian query, quoted using Xapian boolean term quoting
- rules: if the ID contains whitespace or a close paren or
- starts with a double quote, it must be enclosed in double
- quotes and double quotes inside the ID must be
- doubled. The astute reader will notice this is a special
- case of the batch input format for **notmuch-tag(1)**;
- note that the single message-id query is mandatory for
- **notmuch-restore(1)**.