st: matrix operators that return matrices not allowed in this context

[Joe Case Orsini <jorsini@stanford.edu>]

I'm trying to run a monte carlo simulation, and when i run it with one repetition, everything goes through fine. when i run it for 2 repetitions, stata returns the error message in the subject line.
the problem is with the matrix b in the code below, defined using the OLS coefficient estimates. also, if i add drop b to the end of the code, stata returns "b not found" even though it will display b during the loop. I'm sure there is an obvious fix here, but would really appreciate an explanation of what's going wrong. 


			forvalues i=1/`reps'{
			gen eps= rnormal(0,exp(`gamma'*x1))
			mkmat eps
			mat y = yfit + eps
			svmat y
			qui reg y x1 x2 
			predict e, r
			mat b=_b[_cons]\_b[x1]\_b[x2]
			mat list b
			gen e2=e^2
			qui reg e2 x1 x2
			scalar chi2=`T'*e(r2)
			mat CHISTAT`beta'`gamma'=nullmat(CHISTAT`beta'`gamma')\chi2
			
			drop y eps e e2
			mat drop y eps 
	
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