Where does get_exp come from, it isn't mentioned in tagger's documentation?

Just looked at the documentation and it seems there are methods that return hashes with occurrence frequencies ready to use. Especially get_nouns and get_proper_nouns seem to offer just what you want:

"get_proper_nouns TAGGED_TEXT

Given a POS-tagged text, this method returns a hash of all proper nouns and their occurrence frequencies...."