Location is going to gain some members through an implements statement, like

  Location implements Blah;

but we need those imported members from Blah to be [Unforgeable] in addition to all the existing ones on Location.  So let's allow [Unforgeable] directly on interface Location, to indicate that all of its members (including the ones from the implements statement) to be unforgeable.

toString/valueOf on Location also need to be unforgeable.