((x \underset{A}{\Downarrow} ~y \overset{A}{\underset{B}{\Downarrow}}) \underset{B}{\Downarrow} ~z \overset{B}{\underset{C}{\Downarrow}}) \underset{C}{\Downarrow} | ((x \underset{A}{\Downarrow} ~y \overset{A}{\underset{B}{\Downarrow}}) \underset{B}{\Downarrow} ~z \overset{B}{\underset{C}{\Downarrow}}) \underset{C}{\Downarrow} |