The poisson_lpmf
function is built into Stan.
See https://mc-stan.org/docs/2_28/functions-reference/poisson.html for
more details.
cat(poisson_lpmf_stan[["source_code"]])
#> // The `poisson_lpmf` function is built into Stan.
#> // See <https://mc-stan.org/docs/2_28/functions-reference/poisson.html>.