Session doesn't use anymore prover id.
Prover ids are only used for the command line option "-P". The user can choose what he wants (they must be unique) The prover name and version should not be modified. If someone want to test different command line options for a prover he can use the "alternative" field. If someone want to replay an external proof but he doesn't have the corresponding prover (same name,version,alternative), why3ide ask for a replacement among the known provers. The choice can be saved.