<<< native [ ESub (EIdentifier "A") (ESub (EIdentifier "B") (EIdentifier "C")) , ESymbol Rel "=" , ESub (EIdentifier "A") (ESub (EIdentifier "B") (EUnder False (EIdentifier "C") (EPhantom (EIdentifier "D")))) ] >>> tex A_{B_{C}} = A_{B_{\underset{\phantom{D}}{C}}}