<<< native
[ ESubsup
    (EIdentifier "S")
    (ESubsup (EIdentifier "Q") (ENumber "4") (ENumber "3"))
    (ESubsup (EIdentifier "P") (ENumber "2") (ENumber "1"))
, ESymbol Rel "="
, ESubsup
    (EIdentifier "S")
    (ESubsup
       (EIdentifier "Q")
       (EGrouped [ ENumber "4" , EPhantom (EText TextNormal "z") ])
       (ENumber "3"))
    (ESubsup
       (EIdentifier "P")
       (ENumber "2")
       (EGrouped [ ENumber "1" , EPhantom (EText TextNormal "z") ]))
]
>>> tex
S_{Q_{4}^{3}}^{P_{2}^{1}} = S_{Q_{4\phantom{\text{z}}}^{3}}^{P_{2}^{1\phantom{\text{z}}}}