$\begin{array}{ccc}& \stackrel{A}{\to }& \end{array}\begin{array}{ccc}& \underset{A}{\overset{}{\to }}& \end{array}\begin{array}{ccc}& \stackrel{B}{\leftarrow }& \end{array}\begin{array}{ccc}& \underset{B}{\overset{}{\leftarrow }}& \end{array}\begin{array}{ccc}& C\uparrow & \end{array}\begin{array}{ccc}& \uparrow C& \end{array}\begin{array}{ccc}& D\downarrow & \end{array}\begin{array}{ccc}& \downarrow D& \end{array}$