<code>_lpdf</code> and <code>_lpmf</code> (density and mass functions) can be used as probability functions in distribution statements. As with the built-in functions, the first argument will appear on the left of the distribution statement operator (<code>~</code>) in the distribution statement and the other arguments follow. For example, suppose a function returning the log of the density of <code>y</code> given parameter <code>theta</code> allows the use of the distribution statement is defined as follow