<<< native
[ EUnderover
    False
    (EUnderover
       False
       (EIdentifier "BASE")
       (EIdentifier "under-1")
       (EPhantom (EIdentifier "fail")))
    (EIdentifier "under-2")
    (EPhantom (EIdentifier "fail"))
, ESymbol Rel "="
, EUnder
    False
    (EUnder False (EIdentifier "BASE") (EIdentifier "under-1"))
    (EIdentifier "under-2")
]
>>> tex
\underset{under-2}{\overset{\phantom{fail}}{\underset{under-1}{\overset{\phantom{fail}}{BASE}}}} = \underset{under-2}{\underset{under-1}{BASE}}