The invention provides compounds of formula I:wherein R1 to R4 and n have any of the meanings defined in the specification and their pharmaceutically acceptable salts. When a signal generated by operating a power supply key on a remote controller is received, video and audio are muted, a backlight of the LCD panel is turned off, and a power supply to the LCD panel is turned off. The apparatus stores a music-playing data set of a subject music piece for a reference and a music-playing data sets of the stepped lesson, and comprises a music-playing input device for the user to input music-playing manipulations. A detector array includes a plurality of detectors engaged against each other.