module Implementation:sig..end
type 'connection_state t
'connection_state t is something which knows how to respond to one query, given
a 'connection_state. That is, you can create a 'connection_state t by providing
a function which takes a query *and* a 'connection_state and provides a response.
The reason for this is that rpcs often do something like look something up in a
master structure. This way, Implementation.t's can be created without having the
master structure in your hands.
module Description:sig..end
val description : 'a t -> Description.t