8.7 Program Block:
model program block consists of optional variable declarations followed by statements. The variables in the model block are local variables and are not written as part of the output.
Local variables may not be defined with constraints because there is no well-defined way to have them be both flexible and easy to validate.
The statements in the model block typically define the model. This is the block in which probability (sampling notation) statements are allowed. These are typically used when programming in the BUGS idiom to define the probability model.