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