<<< mml $$2^{3^4}=2^{3^{4^{\phantom{\mathrm{fail}}}}}$$ >>> native [ ESuper (ENumber "2") (ESuper (ENumber "3") (ENumber "4")) , ESymbol Rel "=" , ESuper (ENumber "2") (ESuper (ENumber "3") (ESuper (ENumber "4") (EPhantom (EIdentifier "fail")))) ]