<<< native [ ESubsup (EIdentifier "p") (EIdentifier "x") (ENumber "3") , ESymbol Rel "=" , EUnder False (ESubsup (EIdentifier "p") (EIdentifier "x") (ENumber "3")) (EPhantom (EText TextNormal "l")) ] >>> mml $$p_x^3=\underset{\phantom{\mathrm{\text{l}}}}{p_x^3}$$