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