<<< native [ EOver False (EIdentifier "B") (EIdentifier "overscript") , ESymbol Rel "=" , EOver False (EGrouped [ EPhantom (ENumber "1") , EIdentifier "B" , EPhantom (ENumber "1") ]) (EIdentifier "overscript") ] >>> mml $$\stackrel{\mathrm{overscript}}{B}=\stackrel{\mathrm{overscript}}{\phantom{1}B\phantom{1}}$$