初学STATA在聚类稳健做hausman检验时运行总显示invalid syntax，为什么啊？
请各位大侠帮帮忙吧~~

. quietly xtreg  endowment township ,re
. scalar theta=e(theta)
. global yandxforhausman  endowment township unemployment peasantincome decentralization fiscalburden openness market
. sort id
. foreach x of valist $yandxforhausman{
  2. by id:egen mean'x'=mean('x')
  3. gen md'x'='x'-mean'x'
  4. gen red'x'='x'-theta*mean'x'
  5. }
invalid syntax
r(198);