* We currently have three different types of documents (mail, ghost,
* and directory) and also some metadata.
*
+ * There are two kinds of prefixes used in notmuch. There are the
+ * human friendly 'prefix names' like "thread:", which are also used
+ * in the query parser, and the actual prefix terms in the database
+ * (e.g. "G"). The correspondence is maintained in the file scope data
+ * structure 'prefix_table'.
+ *
* Mail document
* -------------
* A mail document is associated with a particular email message. It