The boundary conditions were reversed in Box.Inbox when compared with
Box.Overlaps. Since InBox is not being used and Overlaps is, I went with the
definitions from Overlaps. Also configured unit tests to exercise this
behavior.
I put scare quotes around "correctly" because I can't swear this is the
intended behaviour of the original interpreter.
I don't think accessing filenames that end with / in the .DCPs is even
defined behaviour, so this is a best guess.