code: boolean setSessionType (SessionType sessionType) {
          // allow just once, accept only non-null parameter
          if (sessionType == null || this.sessionType != null) return false;
          this.sessionType = sessionType;
          return true;
      }
docs: Sets the session's type (discovery or operational).
  <p>
  The type may and must be set just once. Repeated calls of this method will fail. <code>true</code> will be
  returned if the session type was set successfully, <code>false</code> if not.
  
  @param sessionType the session type
  @return <code>true</code> if the session type was set successfully, <code>false</code> if not