<<< native
[ ESuper (EIdentifier "a") (ESymbol Bin "+")
, ESymbol Rel "="
, EUnderover
    False
    (ESuper (EIdentifier "a") (ESymbol Bin "+"))
    (EPhantom (EIdentifier "x"))
    (EPhantom (EIdentifier "x"))
]
>>> tex
a^{+} = \underset{\phantom{x}}{\overset{\phantom{x}}{a^{+}}}