<<< native
[ EFraction NormalFrac (ENumber "1") (ENumber "8")
, ESymbol Rel "="
, EFraction
    NormalFrac
    (ENumber "1")
    (EUnder False (ENumber "8") (EPhantom (ENumber "0")))
]
>>> tex
\frac{1}{8} = \frac{1}{\underset{\phantom{0}}{8}}