<<< native
[ ESubsup (EIdentifier "p") (EIdentifier "x") (ENumber "3")
, ESymbol Rel "="
, EUnder
    False
    (ESubsup (EIdentifier "p") (EIdentifier "x") (ENumber "3"))
    (EPhantom (EText TextNormal "l"))
]
>>> tex
p_{x}^{3} = \underset{\phantom{\text{l}}}{p_{x}^{3}}