Register_raw
is like Register
except you get the whole pipe to deal with.
This is useful if, e.g., your caller_model_of_response
function can fail, so
that you'd like to filter items out from the result pipe.
You can use neither dispatch_iter_multi
nor implement_direct_multi
if you
use this, as their non-versioned counterparts do not get access to pipes.
Version_i : sig ... end