Commutative Diagram

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\array{
        \rowopts{\rowalign{center}}
           X & \overset{\ulap{g}}{\longrightarrow}  & Y \\

        \downarrow \rlap{\mathop{id}} & & \llap{\alpha}{\downarrow} \\

        \rowopts{\rowalign{center}}
           X & \underset{\dlap{h}}{\longrightarrow} & Z

      }

\array {      \mathbb{R}        \\
         \downarrow \rlap{\pi}  \\
                  X
       }

More information on these examples may be found in Example: Commutative diagrams (Overlap commands) of the WebTeX documentation.