HoTT/HoTT — pull request #347 by JasonGross (2014-03-29 18:31:10+00:00)
Title: Prove Theorem 7.2.2 from the book

I chose to use the first proof because I couldn't find 4.7.7 already in
the library. I didn't use [dpath_forall] (which HoTTBook.v tells me is
2.9.6) because that requires [Funext], and this lemma doesn't need it.