Task: Question Generation
Input (English): Rebetiko, initially a music associated with the lower classes, later (and especially after the population exchange between Greece and Turkey) reached greater general acceptance as the rough edges of its overt subcultural character were softened and polished, sometimes to the point of unrecognizability. It was the base of the later laïkó (song of the people). The leading performers of the genre include Apostolos Kaldaras, Grigoris Bithikotsis, Stelios Kazantzidis, George Dalaras, Haris Alexiou and Glykeria.
Output (English): Rebetiko was the base of what?