`set` is now a proper type constructor; added operation for set monad
`set` is now a proper type constructor; added operation for set monad

MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle