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