<<< native
[ EArray
    [ AlignCenter , AlignCenter , AlignCenter ]
    [ [ [ EGrouped
            [ EPhantom (EText TextNormal "fail")
            , EUnder
                False (EIdentifier "HT") (EPhantom (EText TextNormal "fail"))
            ]
        ]
      , [ ESymbol Rel "=" ]
      , [ EGrouped
            [ EOver
                False (EIdentifier "TH") (EPhantom (EText TextNormal "fail"))
            , EPhantom (EText TextNormal "fail")
            ]
        ]
      ]
    ]
]
>>> tex
\begin{matrix}
{\phantom{\text{fail}}\underset{\phantom{\text{fail}}}{HT}} & = & {\overset{\phantom{\text{fail}}}{TH}\phantom{\text{fail}}}
\end{matrix}