Register_raw
is like Register
except you get to deal with the whole pipe.
This is useful if, e.g., your model_of_update
function can fail, so that
you'd like to filter items out from the result pipe.
Version_i : sig ... end
val rpc : (Version_i.query, Version_i.state, Version_i.update, Version_i.error) Rpc.State_rpc.t