Updated TIMESTAMP.
Added function `directory-prefix-overlap` with, hopefully, decent semantics. Also added `+empty-pathname+`. Switched `catalog`s to namestrings as keys.
Conditionalized away code referring to unknown asdf:defsystem traversal function.
Added note.
Started working on hierarchical sys/files menu.
Added some functionality.
Added some functionality (cf., DOC-BITS-LOCATIONS).
Minor cosmetic changes.