<<< native
[ EUnder
    False
    (EIdentifier "munder baseline")
    (EIdentifier "munder under script")
, ESymbol Rel "="
, EOver
    False
    (EUnder
       False
       (EIdentifier "munder baseline")
       (EIdentifier "munder under script"))
    (EPhantom (EIdentifier "fail"))
]
>>> tex
\underset{munder\ under\ script}{munder\ baseline} = \overset{\phantom{fail}}{\underset{munder\ under\ script}{munder\ baseline}}