munder-01.test

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