A reference is encoded as the actual reference value, and a symbol describing the state of that actual reference value.

*Reference* __=__ `(` *ReferenceState* String `)`.

The string contains the actual reference value.

*ReferenceState* __=__ `INVALID` __|__ `ZETTEL` __|__ `SELF` __|__ `FOUND` __|__ `BROKEN` __|__ `HOSTED` __|__ `BASED` __|__ `QUERY` __|__ `EXTERNAL`.