<<< native
[ EOver False (EIdentifier "B") (EIdentifier "overscript")
, ESymbol Rel "="
, EOver
    False
    (EGrouped
       [ EPhantom (ENumber "1")
       , EIdentifier "B"
       , EPhantom (ENumber "1")
       ])
    (EIdentifier "overscript")
]
>>> tex
\overset{overscript}{B} = \overset{overscript}{\phantom{1}B\phantom{1}}