+able to be determined from an application's configuration files. Plugin
+should be a function that takes a single required argument ``url``,
+which is a :class:`sqlalchemy.engine.url.URL` with partial
+authentication information, including a database name, and should return either
+``None`` if it was unable to fill in authentication credentials for the
+user, or the completed URL. Mutating the passed argument and then
+returning it is the expected modus operandi of a plugin. The function
+that runs this plugin is :func:`wizard.sql.auth`.