I have two parameters (random variables):

parameters{

vector[3] Alpha;

vector[2] Beta;

…

}

model{

Alpha ~ normal(mu1, var1);

Beta ~ normal(mu2, var2);

…

}

The 5 means and 5 variances are estimated from the data (this is working for me, based on long-format data).

I would like to have another parameter where the mixture (combination/sum) of each combination between alpha and beta is shown.

vector[6] mixture; // 3 times 2=6 combinations

mixture ~ normal(mu1+mu2, var1+ var2 + 2*cov(1,2));

Is this possible within the .stan file or otherwise with the results in R? I was unsure what to look for… A nudge in the right direction is appreciated!