<<< 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}}}