> I don't see how you programatically can easily separate your case
> from e.g.
> 
>> According to your analysis
>>>42 is a nice number
>> you indicate that 42 is a nice number.
> 
> but suggestions are welcome. :)

Not leaving blank lines before and after quotes is not my preferred
style of writing messages. ;-)

> Secondly, RFC 2646 compliant MUAs should never generate this case in
> the first place.  Even if you don't generate RFC 2646 (Gnus doesn't)
> you could QP encode the character to prevent the ambiguity.

Does RFC 2646 really try to solve the problem this way?  In this
case, the RFC is horribly broken.  Attaching semantic to QP encoding
vs. not-encoding is clearly an extremely bad idea, and it certainly
contradicts the spirit of MIME.