u to generate additional quantities
of interest from a fitted model without re-running the sampler.
Instead, you write a modified version of the original Stan program
and add a generated quantities block or modify the existing one
which specifies how to compute the new quantities of interest.
Running the <code>generate_quantities</code> method on the new program
together with sampler outputs (i.e., a set of draws)
from the fitted model runs the generated quantities block
of the new program using the the exi