Collapses the unit-level design to one row per shock, following the Borusyak-Hull-Jaravel (2022) equivalence. With controls partialled out of the outcome and treatment (weighted FWL), each shock \(n\) gets an exposure weight \(s_n=\sum_i e_i s_{in}\) and exposure-weighted means \(\bar y_n=\sum_i e_i s_{in}\tilde y_i/s_n\) and \(\bar x_n\) similarly. Running an IV of \(\bar y_n\) on \(\bar x_n\) with instrument \(g_n\) and weights \(s_n\) reproduces the location-level shift-share estimate exactly (see [ssb_equivalence()]).