* Estimate the model
poisson trade lypex lypim lyex lyim ldist border comlang colony landl_ex landl_im lremot_ex lremot_im comfrt_wto open_wto, robust

* Get fitted values (of the linear index, not of trade)
predict fit, xb

* Square the fitted values
gen fit2=fit^2

* Estimate the model with the additional regressor
poisson trade lypex lypim lyex lyim ldist border comlang colony landl_ex landl_im lremot_ex lremot_im comfrt_wto open_wto fit2, robust

* Test the significance of the additional regressor (this is equivalent to a t-test on fit2)
test fit2=0