Compounds of formula and their pharmaceutically acceptable salts are provided as well as processes for the manufacture of such compounds. By the operation to a demount-operating part, the hook member slidingly moves from a locking position where the hook member engages with an engaging hole of the electronic apparatus, to an unlocking position where the hook member does not engage with the engaging hole. An optical spectrum analyzer is used at the output of the transmission link of interest to accurately measure the output power of each wavelength.