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