Notable for code archaeology: the docstrings weren't in the previous version[].

(He also switched from manually implementing counting behavior with a Dict to a Counter, added the P function which replaces an inline dict lookup, similarly added the candidates function, changed the somewhat awkward known_edits2 function to just edits2, and reorded some things.)

[] http://web.archive.org/web/20160408180602/http://norvig.com/...

edit: small edits