I feel silly, but I can’t get this to work:

```
functions {
matrix sparse_matrix(){
int m = 2;
vector[3] a = [1, 2, 3]';
int b[3] = {1, 2, 2};
int c[3] = {1, 1, 2};
matrix[m, m] d = csr_to_dense_matrix(m, m, a, b, c);
return(d);
}
}
```

Which I thought would give me the matrix d = \begin{bmatrix} 1 & 2 \\ 0 & 3\end{bmatrix} but instead I get:

```
> expose_stan_functions("model/csr_matrix.stan")
> sparse_matrix()
Error in sparse_matrix() :
Exception: csr_to_dense_matrix: u/z (1) and v (3) must match in size (in 'unknown file name' at line 8)
```