code: Supervisor commonSupervisorOr(final Class<?> protocol, final Supervisor defaultSupervisor) {
      final Supervisor common = commonSupervisors.get(protocol);
  
      if (common != null) {
        return common;
      }
  
      return defaultSupervisor;
    }
docs: Answers the common Supervisor for the given protocol or the defaultSupervisor if there is
  no registered common Supervisor. (INTERNAL ONLY)
  @param protocol the {@code Class<?>} protocol to supervise
  @param defaultSupervisor the Supervisor default to be used if there is no registered common Supervisor
  @return Supervisor