<<< mml $$A_B^C=A_B^C\phantom{\frac{\mathrm{test}}{\mathrm{failed}}}$$ >>> native [ ESubsup (EIdentifier "A") (EIdentifier "B") (EIdentifier "C") , ESymbol Rel "=" , ESubsup (EIdentifier "A") (EIdentifier "B") (EIdentifier "C") , EPhantom (EFraction NormalFrac (EIdentifier "test") (EIdentifier "failed")) ]