I'm defining a Multinomial-Dirichlet model in JAGS and want to assign some hyperpriors to the parameters of the Dirichlet distribution. In the WinBugs manual I read that "the parameters of Dirichlet and Wishart distributions and the order (N) of the multinomial distribution must be specified and cannot be given prior distributions. (There is, however, a trick to avoid this constraint for the Dirichlet distribution). What's this trick?