Hi! is my first comment on the group. I want to congratulate everyone for a such a remarkable work!

This is my question:

I want to build a customised likelihood following this paper:

https://link.springer.com/article/10.3758/s13428-016-0746-9

I want to use the built-in gamma distribution in my function but I don’t know if it is legal, the PARSER shows the

error : expression not assignable to the left side. You will forgive me for not giving the full script but I attach the function in particular!

```
real ptcm_log(vector yobs, vector theta, matrix X, real alpha, real sigma){
vector[num_elements(yobs)] lpdf;
vector[num_elements(yobs)] pdf;
vector[num_elements(yobs)] cdf;
real term1;
real term2;
vector[num_elements(yobs)] prob;
vector[num_elements(yobs)] p;
real lprob;
p = exp(-exp(X * theta));
lpdf = gammal_lpdf(yobs | alpha, beta);
pdf = exp(lpdf);
cdf = gamma_cdf(yobs , alpha, beta);
for(i in 1:num_elements(yobs)) {
term1 = (-log(p[i])*pdf[i]);
term2 = exp(log(p[i])*cdf[i]);
prob[i] = term1 * term2;
}
lprob = sum(log(prob));
return lprob;
}
```