<<< native
[ EOver False (EIdentifier "BASE") (EIdentifier "over script")
, ESymbol Rel "="
, EOver
    False
    (EOver
       False
       (EOver False (EIdentifier "BASE") (EIdentifier "over script"))
       (EPhantom (EIdentifier "over")))
    (EPhantom (EIdentifier "over"))
]
>>> tex
\overset{over\ script}{BASE} = \overset{\phantom{over}}{\overset{\phantom{over}}{\overset{over\ script}{BASE}}}