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