Duality in Intuitionistic Propositional Logic

It is known that provability in propositional intuitionistic logic is Pspace-complete. As Pspace is closed under complements, there must exist a Logspace-reduction from refutability to provability. Here we describe a direct translation: given a formula φ, we define ̅φ so that ̅φ is provable if and only if φ is not.