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