<<< mml $$U_t=U_t\phantom{\frac{\mathrm{test}}{\mathrm{failed}}}$$ >>> native [ ESub (EIdentifier "U") (EIdentifier "t") , ESymbol Rel "=" , ESub (EIdentifier "U") (EIdentifier "t") , EPhantom (EFraction NormalFrac (EIdentifier "test") (EIdentifier "failed")) ]