Internal functions for the Cross-Sectionally augmented Distributed Lag
(CS-DL) estimator of Chudik, Mohaddes, Pesaran & Raissi (2016). The CS-DL
regression is
$$\Delta y_{it} = \alpha_i + w'_i x_{it}
+ \sum_{\ell=0}^{p_x} \phi'_{i\ell} \Delta x_{i,t-\ell}
+ \delta'_i \bar{z}_t + u_{it},$$
where the long-run coefficient \(w_i\) is identified directly as the
coefficient on the level of \(x_{it}\). This file provides helpers to
augment a formula with the required \(\Delta x_{t-\ell}\) terms before
the main dcce() pipeline runs.