Merge pull request #130 from stan-dev/bugfix/empty-model-block

The model block is not mandatory, can be omitted if empty

Author: mitzimorris

src/reference-manual/mcmc.Rmd

@@ -531,8 +531,10 @@ large as necessary.
 In some situations, such as pure forward data simulation in a directed
 graphical model (e.g., where you can work down generatively from known
 hyperpriors to simulate parameters and data), there is no need to
-declare any parameters in Stan, the model block will be empty, and all
-output quantities will be produced in the generated quantities block.
+declare any parameters in Stan, the model block will be empty (and thus
+can be omitted), and all output quantities will be produced in the
+generated quantities block.
+
 For example, to generate a sequence of $N$ draws from a binomial with
 trials $K$ and chance of success $\theta$, the following program suffices.
 
@@ -542,20 +544,16 @@ data {
   int<lower=0> K;
   int<lower=0> N;
 }
-model {
-}
 generated quantities {
   int<lower=0,upper=K> y[N];
   for (n in 1:N)
     y[n] = binomial_rng(K, theta);
 }
 ```
 
-This program includes an empty model block because every Stan program
-must have a model block, even if it's empty.  For this model, the
-sampler must be configured to use the fixed-parameters setting because
-there are no parameters.  Without parameter sampling there is no need
-for adaptation and the number of warmup iterations should be set to
+For this model, the sampler must be configured to use the fixed-parameters
+setting because there are no parameters.  Without parameter sampling there
+is no need for adaptation and the number of warmup iterations should be set to
 zero.
 
 Most models that are written to be sampled without parameters will not