在winbugs中，前面几步都没问题，但是update的时候，就提示 cannot bracket slice for node tt ，请问怎么能解决呢？
下面是程序：
model
{
for(i in 1:N){
r[i]~dnegbin(u[i],t)
b[i]~dnorm(0.0,tau)
logit(u[i])<-alpha0+alpha1*x1[i]+alpha2*x2[i]+alpha3*x3[i]+alpha4*x4[i]+alpha5*x5[i]+b[i]
x4[i]<-alpha41*x41[i]+alpha42*x42[i]+alpha43*x43[i]+alpha44*x44[i]
x5[i]<-alpha51*x51[i]+alpha52*x52[i]+alpha53*x53[i]+alpha54*x54[i]
}
alpha0~dnorm(0.0,1.0E-6)
alpha1~dnorm(0.0,1.0E-6)
alpha2~dnorm(0.0,1.0E-6)
alpha3~dnorm(0.0,1.0E-6)
alpha4~dnorm(0.0,1.0E-6)
alpha5~dnorm(0.0,1.0E-6)
alpha41~dnorm(0.0,1.0E-6)
alpha42~dnorm(0.0,1.0E-6)
alpha43~dnorm(0.0,1.0E-6)
alpha44~dnorm(0.0,1.0E-6)
alpha51~dnorm(0.0,1.0E-6)
alpha52~dnorm(0.0,1.0E-6)
alpha53~dnorm(0.0,1.0E-6)
alpha54~dnorm(0.0,1.0E-6)
sigma~dunif(0,100)
tau<-1/(sigma*sigma)
tt~dgamma(1000,1000)
t<-1/tt
}
list(N=240,
r=c(0,0,0,0,0,0,0,0,3,0,1,4,2,2,2,3,1,1,2,3,5,3,0,0,0,0,0,0,0,0,0,4,6,9,9,3,11,9,9,5,5,10,13,10,5,2,0,0,0,0,0,0,0,0,0,0,2,1,2,3,1,2,3数据太长了就不贴了
list(alpha0=0,alpha1=0,alpha2=0,alpha3=0,alpha4=0,alpha5=0,alpha41=0,alpha42=0,alpha43=0,alpha44=0,alpha51=0,alpha52=0,alpha53=0,alpha54=0,sigma=1,tt=1)