<<< native
[ ESuper (EIdentifier "S") (ENumber "3")
, ESymbol Rel "="
, ESuper (EIdentifier "S") (ENumber "3")
, EPhantom
    (EFraction NormalFrac (EIdentifier "test") (EIdentifier "failed"))
]
>>> tex
S^{3} = S^{3}\phantom{\frac{test}{failed}}