In the documentation for Format Strings
(http://docs.python.org/dev/3.0/library/string.html#formatstrings), the
grammar has the following rule:

> conversion        ::=  "r" | "s"

However, the documentation later reads:

> Three conversion flags are currently supported: '!s' which calls str()
on the value, '!r' which calls repr() and '!a' which calls ascii().

This implies that the correct rule for the grammar is:

> conversion        ::=  "a" | "r" | "s"