Congruence permutable MS-algebras

We characterize the congruence permutable MS-algebras in a result analogous to that of Nachbin for distributive lattices. In our main result we prove, among other things, that the congruence permutability of an MS-algebra L is equivalent to the existence of two binary operations in L which satisfy certain equational properties. We show that the class of all congruence permutable MS-algebras with these additional operations forms an equational class. Finally we give a Mal'cev term for this new variety.