Chapovsky, Yu., and A. A. Kalyuzhnyi. “Factorization of Conditional Expectations on Kac Algebras and Quantum Double Coset Hypergroups”.
Ukrains’kyi Matematychnyi Zhurnal, vol. 55, no. 12, Dec. 2003, pp. 1669-77,
https://umj.imath.kiev.ua/index.php/umj/article/view/4029.