Starting working on the Hopf fibration
Merge branch 'master' of https://github.com/HoTT/book
Merge branch 'master' of https://github.com/HoTT/book
Merge branch 'master' of https://github.com/HoTT/book
Merge branch 'master' of https://github.com/HoTT/book
Merge branch 'master' of https://github.com/HoTT/book
Merge branch 'master' of https://github.com/HoTT/book
Merge branch 'master' of https://github.com/HoTT/book
continue Hopf fibration