Hello everyone,

I want to use the function ‘diag_matrix’ to create a diagonal matrix by indexing one row from a matrix, for example, I have a matrix

```
matrix[n,p] X;
```

and I want to create n diagonal matrix (with a dimension of p*p) that uses each row of the matrix X as diagonal entries for computing such as for i in (1:n)

```
diag_matrix(X [i,]) ;
```

but it seems the stan did not allow the above syntax. Anyway to solve this? Thanks!