Proposal #3339: Add (+>) as a synonym for mappend

[Malcolm Wallace]

> Aside: I don't think we should try to avoid collisions with downstream symbols when growing the base libraries. It's just not a scalable approach engineering wise. We should use namespaces to distinguish symbols from different packages.

Agreed.  But in this particular case we should remember that pretty was once part of base, and remains a ghc boot package.

Regards,
    Malcolm