<<< native
[ EUnder False (EIdentifier "middle") (EIdentifier "bottom")
, ESymbol Rel "="
, EUnderover
    False
    (EIdentifier "middle")
    (EIdentifier "bottom")
    (EPhantom (EIdentifier "fail"))
]
>>> tex
\underset{bottom}{middle} = \underset{bottom}{\overset{\phantom{fail}}{middle}}