Proposal: Num instance for tuples

[Henning Thielemann]

On Thu, 18 Oct 2012, Brent Yorgey wrote:

> I am +1 for this proposal, but only as long as these instances are not
> exported by the Prelude.  They are useful when you know you want them
> but will confuse newbies -- based on evidence from #haskell, where
> lambdabot has such instances in scope.

Please no orphan instances - I think a pair type that supports arithmetics 
deserves a custom type.