1. Create the dummies and estimate
levelsof year, local(time)
foreach t of local time {
local y_rr = `t' - 1975
gen intra_nafta`t' = yr`y_rr' * intra_nafta
gen imp_nafta_rest`t' = yr`y_rr' * imp_nafta_rest
}

xtreg lnV lncGDP lnpGDP lnD yr* intra_nafta* imp_nafta_rest*, fe vce(robust)

Who can help me explain how y_rr is defined in the above code? What meaning of yr`y_rr' * intra_nafta ? Thank you!