<<< mml $$\genfrac{}{}{0.2ex}{}{\frac{\stackrel{\phantom{\text{!}}}{a}}{b}}{\frac{c}{d}}=\genfrac{}{}{.2ex}{}{\frac{a}{b}}{\frac{c}{\underset{\phantom{\text{!}}}{d}}}$$ >>> 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 "!")))) ]