Initialization code for typeclasses, setting up the default tactic
for instance search.
Author: Matthieu Sozeau
Institution: LRI, CNRS UMR 8623 - University Paris Sud
Hints for the proof search: these combinators should be considered rigid.
Apply using the same opacity information as typeclass proof search.
The unconvertible typeclass, to test that two objects of the same type are
actually different.