The Son is mutable and alterable while the Father is not. The resulting light signal is kept constant by controlling the alterable cuff pressure. The light-signal PG shall become zero by controlling the alterable pressure in the cuff. Bridge height may be fixed or alterable. The productivity model Saari is the only model with alterable characteristics. The discretion was pre-eminently theirs, alterable only on the grounds mentioned above. Once established, these abnormal behaviours in birds are often not alterable. Alterable ground clearance allows easier transportation in an airplane.