castle                  Data from Cheng and Hoekstra (2013)
df_het                  Simulated data with two treatment groups and
                        heterogenous effects
df_hom                  Simulated data with two treatment groups and
                        homogenous effects
did2s                   Calculate two-stage difference-in-differences
                        following Gardner (2021)
event_study             Estimate event-study coefficients using TWFE
                        and 5 proposed improvements.
gen_data                Generate TWFE data
get_honestdid_obj_did2s
                        get_honestdid_obj_did2s
honest_did_did2s        honest_did_did2s
robust_solve_XtX        Robust solve for X'X beta = X'Y using QR
                        decomposition
