<<< native
[ ESubsup (EIdentifier "T") (EIdentifier "m") (EIdentifier "n")
, ESymbol Rel "="
, EOver
    False
    (ESubsup (EIdentifier "T") (EIdentifier "m") (EIdentifier "n"))
    (EPhantom (EText TextNormal "x"))
]
>>> tex
T_{m}^{n} = \overset{\phantom{\text{x}}}{T_{m}^{n}}