<<< mml $$L_{X_h}=L_{X_{h_{\phantom{\mathrm{fail}}}}}$$ >>> native [ ESub (EIdentifier "L") (ESub (EIdentifier "X") (EIdentifier "h")) , ESymbol Rel "=" , ESub (EIdentifier "L") (ESub (EIdentifier "X") (ESub (EIdentifier "h") (EPhantom (EIdentifier "fail")))) ]