<<< mml $$A_{B_{C_D}}=A_{B_{C_{D_{\phantom{E}}}}}$$ >>> native [ ESub (EIdentifier "A") (ESub (EIdentifier "B") (ESub (EIdentifier "C") (EIdentifier "D"))) , ESymbol Rel "=" , ESub (EIdentifier "A") (ESub (EIdentifier "B") (ESub (EIdentifier "C") (ESub (EIdentifier "D") (EPhantom (EIdentifier "E"))))) ]