Why add to the head and tail of the motion string?

[SatireY]

motion_string.append( (f'<motion_id_{self.m_codebook_size}>' + ''.join([f'<motion_id_{int(i)}>' for i in motion_list]) + f'<motion_id_{self.m_codebook_size + 1}>')) Why add it? <motion_id_{self.m_codebook_size}>; <motion_id_{self.m_codebook_size + 1}>