<<< mml $$\underset{\mathrm{long\; under\; script}}{\mathrm{B}}=\underset{\mathrm{long\; under\; script}}{\phantom{\text{x}}\mathrm{B}\phantom{\text{x}}}$$ >>> 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") ]