Minor Description
Currently the UserInitializer creates a rep:principalName index with two declaring node type names: rep:User and rep:Group.
This allows query to make use of the index when they select the user or group node type, but the query engine cannot use this index when the select is for rep:Authorizable.
Adding rep:Authorizable will do the trick even though it looks a bit strange, since rep:Authorizable is an abstract node type and you cannot create a node with this type. However it allows the query engine to leverage the index for rep:Authorizable and is correct in this case. rep:User and rep:Group are the only sub node types of rep:Authorizable, which means the index will contain all rep:Authorizable.