msubsup-01.test

<<< 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") ])
]
>>> tex
R_{0}^{4} = R_{0\phantom{fail}}^{4\phantom{fail}}