munder-10.test

<<< native
[ EUnder False (EIdentifier "B") (EIdentifier "long under script")
, ESymbol Rel "="
, EUnder
    False
    (EGrouped
       [ EPhantom (EText TextNormal "x")
       , EIdentifier "B"
       , EPhantom (EText TextNormal "x")
       ])
    (EIdentifier "long under script")
]
>>> tex
\underset{long\ under\ script}{B} = \underset{long\ under\ script}{\phantom{\text{x}}B\phantom{\text{x}}}