Patch #744877: Explain filter in terms of list comprehension. Remove explanation of int in terms of string.atoi. Explain sum in terms of reduce.
Patch #744877: Explain filter in terms of list comprehension. Remove
explanation of int in terms of string.atoi. Explain sum in terms of
reduce.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator