Slim instance declaration

This is an excerpt from "Disclipline for re-use: Slim instances" http://www.haskell.org/pipermail/libraries/2006-September/005791.html

All instance declarations should be slim.

This means

• Instance declarations (and default definitions) should never contain non-trivial function definitions.
• Instance declarations (and default definitions) should only provide "plumbing" to make existing functions accessible via the type class resolution mechanism.
• The "plumbed" functions should always be exported (since instances are always exported).

Since the naming will not always be straight-forward, the last point is particularly important and would enable more re-use and less re-invention of mostly trivial wheels.