<<< native
[ ESubsup (EIdentifier "p") (EIdentifier "x") (ENumber "3")
, ESymbol Rel "="
, EUnder
    False
    (ESubsup (EIdentifier "p") (EIdentifier "x") (ENumber "3"))
    (EPhantom (EText TextNormal "l"))
]
>>> eqn
p sub x sup 3 = {p sub x sup 3} from {hphantom {roman "l"}}