Amalgamation property for Heyting algebras?

Michael Makkai has shown that something called the Craig Interpolation lemma is equivalent to fact that in Heyting algebras a pushout of an injection is an injection. Now it is easy to show that a pushout of an injection with a quotient map is an injection, so the question is reduced to the amalgamation property. Anyone have an easy proof of that? Michael 444444444444444444444444444444444444