<<< 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") ]))
]
>>> eqn
S sub {Q sub 4 sup 3} sup {P sub 2 sup 1} = S sub {Q sub {4 hphantom {roman "z"}} sup 3} sup {P sub 2 sup {1 hphantom {roman "z"}}}