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