<<< native
[ EArray
    [ AlignCenter , AlignCenter , AlignCenter ]
    [ [ [ ENumber "1" ]
      , [ ESymbol Accent "\8594" ]
      , [ ENumber "2" ]
      ]
    , [ [ ESymbol Rel "\8595" ]
      , [ EGrouped
            [ EIdentifier "x"
            , ESymbol Bin "+"
            , EFraction
                NormalFrac (ENumber "2") (ESuper (EIdentifier "y") (ENumber "3"))
            ]
        ]
      , [ ESymbol Rel "\8595" ]
      ]
    , [ [ ENumber "3" ]
      , [ ESymbol Accent "\8594" ]
      , [ ENumber "4" ]
      ]
    ]
]
>>> tex
\begin{matrix}
1 & \rightarrow & 2 \\
 \downarrow & {x + \frac{2}{y^{3}}} & \downarrow \\
3 & \rightarrow & 4
\end{matrix}