Transformations created with premap are stateless, they depend only on the current input. So one can't build UTF8 decoders with them, for example. That said, if

Transformations created with premap are stateless, they depend only on the current input. So one can't build UTF8 decoders with them, for example. That said, if you supply a function to premap, the result typechecks as a "Transduction".