var
    biopsies[ns,4], #  grades observed in ith session (multinomial)
    nbiops[ns],     # total number of biopsies in ith session
    true[ns],       # true state in ith session
    error[4,4],     # error matrix in taking biopsies
    prior[4,4],     # prior parameters for rows of error[,]
    p[4];           # underlying   incidence of true  states
model {
   for (i in 1:ns){
      true[i]       ~ dcat(p[]);
      biopsies[i,]  ~ dmulti(error[true[i],],nbiops[i]); 
   }

   for (j in 1:4) {
      error[j,]     ~ ddirch(prior[j,]);
   }

   p[]       ~ ddirch(prior[4,]);     # prior for p
}