<<< native
[ ESubsup
    (EIdentifier "A")
    (ESubsup (EIdentifier "C") (ENumber "4") (ENumber "3"))
    (ESubsup (EIdentifier "B") (ENumber "2") (ENumber "1"))
, ESymbol Rel "="
, ESubsup
    (EIdentifier "A")
    (ESubsup
       (EIdentifier "C")
       (ESub (ENumber "4") (EPhantom (EText TextNormal "l")))
       (ENumber "3"))
    (ESubsup
       (EIdentifier "B")
       (ENumber "2")
       (ESuper (ENumber "1") (EPhantom (EText TextNormal "s"))))
]
>>> eqn
A sub {C sub 4 sup 3} sup {B sub 2 sup 1} = A sub {C sub {4 sub {hphantom {roman "l"}}} sup 3} sup {B sub 2 sup {1 sup {hphantom {roman "s"}}}}