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