<<< native
[ ESubsup (EIdentifier "L") (EIdentifier "x") (ENumber "2")
, ESymbol Rel "="
, ESubsup
    (EIdentifier "L")
    (ESub (EIdentifier "x") (EPhantom (EIdentifier "z")))
    (ESuper (ENumber "2") (EPhantom (EIdentifier "z")))
]
>>> eqn
L sub x sup 2 = L sub {x sub {hphantom z}} sup {2 sup {hphantom z}}