<<< native
[ EFraction
    NormalFrac
    (EFraction
       NormalFrac
       (EOver False (EIdentifier "a") (EPhantom (EText TextNormal "!")))
       (EIdentifier "b"))
    (EFraction NormalFrac (EIdentifier "c") (EIdentifier "d"))
, ESymbol Rel "="
, EFraction
    NormalFrac
    (EFraction NormalFrac (EIdentifier "a") (EIdentifier "b"))
    (EFraction
       NormalFrac
       (EIdentifier "c")
       (EUnder False (EIdentifier "d") (EPhantom (EText TextNormal "!"))))
]
>>> tex
\frac{\frac{\overset{\phantom{\text{!}}}{a}}{b}}{\frac{c}{d}} = \frac{\frac{a}{b}}{\frac{c}{\underset{\phantom{\text{!}}}{d}}}