<<< 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") ] >>> mml $$\underset{\mathrm{underscript}}{\overset{\mathrm{overscript}}{B}}=\stackrel{\mathrm{overscript}}{\underset{\mathrm{underscript}}{\phantom{1}B\phantom{1}}}$$