<<< native
[ EUnderover
    False
    (EIdentifier "BASE")
    (EIdentifier "under")
    (EIdentifier "over")
, ESymbol Rel "="
, EOver
    False
    (EUnder False (EIdentifier "BASE") (EIdentifier "under"))
    (EIdentifier "over")
]
>>> tex
\underset{under}{\overset{over}{BASE}} = \overset{over}{\underset{under}{BASE}}