Solution to Exc. 3.9
Lemma 3.3.2,3
fixed misnomers in solution of Exc. 3.9
Added Book_3_3_2,3
simplified 3.9, Import HoTTBook can be a problem
reverted to d7fa7f...
Lemmas 3.3.2 and 3.3.3 added
Backrefs for Lemmas 3.3.2 and 3.3.3 added
Solution to Exc. 3.9
Many improvements in the solution of Exc.3.9
all_to_empty_isequiv with hack
Many small fixes concerning Exc. 3.9
quotient_rect requires its motive to be an hset; close #551
Merge pull request #543 from marcbezem/master

Lemmas 3.3.2 and 3.3.3 and Exc. 3.9
Merge pull request #552 from mikeshulman/quotient

quotient_rect requires its motive to be an hset; close #551
Break a loop that shows up with Keyed Unification

Without keyed unification, `rewrite` inferred that a particular endpoint
of a particular path was `((1 ∘ x ∘ 1) x2)`.  However, in the goal, this
endpoint was passed as (the judgmentally equivalent) `x2`.  So after
`rewrite` finished filling in its holes, it couldn't find the subterm
anymore, breaking the loop.  With keyed unification, it can find the
subterm, so the rewrite loops.
Factor out transport_to_ap
Merge pull request #554 from JasonGross/break-keyed-unification-loop

Break a loop that shows up with Keyed Unification