  In this paper, we show that those sequential products which were proposed by
Liu and Shen and Wu in [J. Phys. A: Math. Theor. {\bf 42}, 185206 (2009), J.
Phys. A: Math. Theor. {\bf 42}, 345203 (2009)] are just unitary equivalent to
the sequential product $A\circ B=A^{\frac{1}{2}}BA^{\frac{1}{2}}$.
