<<< native
[ ESuper (ENumber "2") (ESuper (ENumber "3") (ENumber "4"))
, ESymbol Rel "="
, ESuper
    (ENumber "2")
    (ESuper
       (ENumber "3")
       (ESuper (ENumber "4") (EPhantom (EIdentifier "fail"))))
]
>>> tex
2^{3^{4}} = 2^{3^{4^{\phantom{fail}}}}