Library Coq.Classes.Init
Initialization code for typeclasses, setting up the default tactic
for instance search.
Apply using the same opacity information as typeclass proof search. 
Ltac class_apply c := autoapply c using typeclass_instances.
The unconvertible typeclass, to test that two objects of the same type are
   actually different. 
Class Unconvertible (A : Type) (a b : A) := unconvertible : unit.
Ltac unconvertible :=
match goal with
| |- @Unconvertible _ ?x ?y => unify x y with typeclass_instances ; fail 1 "Convertible"
| |- _ => exact tt
end.
Hint Extern 0 (@Unconvertible _ _ _) => unconvertible : typeclass_instances.
