Basically, the idea is to parse the result of the registred ('s ->
string) function.

For instance if  the user define 'v and 'v printer as follows:

 type state = { a : int ; b : float }
 let state_to_string s = Printf.sprintf "x=%d y=%f" s.v1 s.v2

pid_x and pid_y will be used as output var names in the RIF outputs.

If the user provide no name:
 let state_to_string s = Printf.sprintf "%d %f" s.v1 s.v2

some names will be invented (pid_v0 and pid_v1)