[Haskell-cafe] Non-exported type classes?

What is the effect of declaring a class in a module and not exporting it? Would that prevent to add more instances to that class other than those already defined in the module? More in detail, consider this module: === module Peano (Z,S,C) where data Z data S a -- Not exported class Peano b where instance Peano Z where instance Peano a => Peano (S a) where -- Exported class Peano a => C a where === Would that limit the instances of the class C to the Peano type naturals? Thanks, Roberto Zunino.