Thanks again for the excellent comments.

__init__: good catch.

repr(Rational): The rule for repr is "eval(repr(object)) == object".
Unfortunately, that doesn't decide between the two formats, since both
assume some particular import statements. I picked the one more likely
to be unique, and I assume Decimal picked the shorter one. I can go
either way.

_gcd's sign: It's a happy accident for me. Possibly Sjoerd Mullender
designed it that way. I've added a comment and a test.

__ceil__: I like that implementation better.

2-argument round: Fixed and tested.

equality: Very good point. I've stolen the sandbox code and added
Rational.from_float() using it. I think I also need to make this change
to the comparisons.

hashing: oops, yes these should be hashable. Decimal cheats by comparing
!= to even floats that it's equal to, so I'm going to assume that they
also want Rational(5,2) != Decimal('2.5').

The new patch is against 2.6.