Systems and methods are disclosed to generate a codebook for channel state information by generating a random codebook; partitioning channel state information into a set of nearest neighbors for each codebook entry based on a distance metric; and updating the codebook by finding a centroid for each partition. The ladder comprises a plurality of sections which telescope open and closed, and from a bottom section each section telescopes open to positively lock to the section below it with plungers that lock and unlock each section in sequence as the sections open and close, respectively. The auxiliary nozzle is rotatably mounted on the top end of the main body and is provided with an inclined edge forming adjustably a gap in conjunction with an inclined guide edge of a water chamber of the main body. The backrest structure can have a highly simplified design due to the longitudinal guide being an integral part of the headrest and the swiveling axis of the guiding part being fixed relative to the backrest, and since the frame of the backrest does not require any reinforcing structures or guide rails for accommodating a headrest.