Compound monads in specification languages

We consider the language of "extended subsitutions" involving both angelic and demonic choice. For other related languages expressing program semantics the implicit model of computationis based on a combination of monads by a distributive law. We show how the model of computation underlying extended subsitutions is based on a monad which, while not being a compound monad, has strong similarities to a compound monad based on a distributive law. We discuss these compound monads and monad morphisms between them. We have used the theorem prover Isabelle to formal ise and machine-check our results