Add a clarifying paragraph to PEP 0479, and another post-history date.

pep-0479.txt

@@ -8,7 +8,7 @@ Type: Standards Track
 Content-Type: text/x-rst
 Created: 15-Nov-2014
 Python-Version: 3.5
-Post-History: 15-Nov-2014
+Post-History: 15-Nov-2014, 19-Nov-2014
 
 
 Abstract
@@ -179,6 +179,20 @@ at the time of writing only two responses have been received: one was
 in favor of changing list comprehensions to match generator
 expressions (!), the other was in favor of this PEP's main proposal.
 
+The existing model has been compared to the perfectly-acceptable
+issues inherent to every other case where an exception has special
+meaning.  For instance, an unexpected ``KeyError`` inside a
+``__getitem__`` method will be interpreted as failure, rather than
+permitted to bubble up.  However, there is a difference. Dunder
+methods use ``return`` to indicate normality, and ``raise`` to signal
+abnormality; generators ``yield`` to indicate data, and ``return`` to
+signal the abnormal state.  This makes explicitly raising
+``StopIteration`` entirely redundant, and potentially surprising.  If
+other dunder methods had dedicated keywords to distinguish between
+their return paths, they too could turn unexpected exceptions into
+``RuntimeError``; the fact that they cannot should not preclude
+generators from doing so.
+
 
 References
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