<<< mml $$R_0^4=R_{0\phantom{\mathrm{fail}}}^{4\phantom{\mathrm{fail}}}$$ >>> native [ ESubsup (EIdentifier "R") (ENumber "0") (ENumber "4") , ESymbol Rel "=" , ESubsup (EIdentifier "R") (EGrouped [ ENumber "0" , EPhantom (EIdentifier "fail") ]) (EGrouped [ ENumber "4" , EPhantom (EIdentifier "fail") ]) ]