求助啊啊
openbugs做ARMA（1，1）突变点贝叶斯推断，一直报错：multiple definitions of node xt[1]

代码
library(R2OpenBUGS)
N=1000
xt<-c(x,y)
e<-c(e1,e2)
data<-list("N","xt","e")
parameters<-c("k","alpha","sigma","beta")
inits<-function()
{
  list(k=500,alpha=c(0.2,0.6),beta=c(0.4,0.8),sigma=c(0.1,0.1))
}
model<-function(){
  xt[1]<-0.01
  k~dunif(1,N)
  for(i in 1:N){
    e[i]~dnorm(0,sigma[J[i]])
    xt[i]<-alpha[J[i]]*xt[i-1]+beta[J[i]]*e[i-1]+e[i]
    J[i]<-1+step(i-k)
  }
  for(j in 1:2){
    alpha[j]~dnorm(0,0.001)
    beta[j]~dnorm(0,0.001)
    sigma[j]~dnorm(0,0.001)
  }

}
write.model(model,"model.txt")
model.sim<-bugs(data,inits,parameters,"model.txt",n.chains = 3,n.iter = 10000,debug=TRUE)



虽然觉得做伸手党不好，但是已经卡了好几天了，要死了TAT
求助大神