Upgrade to coverage 3.7.1 and have it auto-build itself on first use.

third_party/coverage-3.7.1/doc/branch.rst

 .. _branch:
 
 ===========================
 Branch coverage measurement
 ===========================
 
 :history: 20091127T201300, new for version 3.2
 :history: 20100725T211700, updated for 3.4.
 :history: 20110604T181700, updated for 3.5.
 :history: 20111214T181800, Fix a bug that Guido pointed out.
@@ skipping 55 matching lines @@
 
 The idea of tracking how lines follow each other was from `Titus Brown`__.
 Thanks, Titus!
 
 __ http://ivory.idyll.org/blog
 
 
 Excluding code
 --------------
 
-If you have :ref:`excluded code <excluding>`, a condtional will not be
-counted as a branch if one of its choices is excluded::
+If you have :ref:`excluded code <excluding>`, a condtional will not be counted
+as a branch if one of its choices is excluded::
 
     def only_one_choice(x):
         if x:
             blah1()
             blah2()
         else:       # pragma: no cover
             # x is always true.
             blah3()
 
-Because the ``else`` clause is excluded, the ``if`` only has one possible
-next line, so it isn't considered a branch at all.
+Because the ``else`` clause is excluded, the ``if`` only has one possible next
+line, so it isn't considered a branch at all.
 
 
 Structurally partial branches
 -----------------------------
 
 Sometimes branching constructs are used in unusual ways that don't actually
 branch.  For example::
 
     while True:
         if cond:
             break
         do_something()
 
 Here the while loop will never exit normally, so it doesn't take both of its
 "possible" branches.  For some of these constructs, such as "while True:" and
 "if 0:", coverage.py understands what is going on.  In these cases, the line
 will not be marked as a partial branch.
 
 But there are many ways in your own code to write intentionally partial
 branches, and you don't want coverage.py pestering you about them.  You can
 tell coverage.py that you don't want them flagged by marking them with a
 pragma::
 
     i = 0
     while i < 999999999:    # pragma: no branch
         if eventually():
             break
 
 Here the while loop will never complete because the break will always be taken
-at some point.  Coverage.py can't work that out on its own, but the
-"no branch" pragma indicates that the branch is known to be partial, and
-the line is not flagged.
+at some point.  Coverage.py can't work that out on its own, but the "no branch"
+pragma indicates that the branch is known to be partial, and the line is not
+flagged.