* center predictors read and z_director
summarize prd
generate cprd = prd - r(mean)
summarize director
generate cdirector = director - r(mean)
* create interaction term
gen rw = cprd*cdirector
* do regression
xi3: regress F.roa cprd cdirector rw
* get mean and sd for computing 1sd below mean, mean, 1sd above mean
summarize cdirector
local mdirector  = r(mean)
local sddirector = r(sd)
* create "yhat1", predicted values at 1sd below mean
local director_at = `mdirector'-`sddirector'
replace rw = cprd*`director_at'
请问，我要用STATA画出prd跟director这两个变量的interaction graph，用的是以上的这些命令，可是在
local director_at = `mdirector'-`sddirector'
replace rw = cprd*`director_at'
这两条的时候老是出现“invalid syntax”，这是为什么啊？