I’m trying to write a function of map_rect. I need to pack up parameters for the theta term and its proving a little troublesome.

The variables are:

```
matrix[NvarsY, N_ind] beta0_ind;
matrix[NvarsX, N_ind] beta_ind [NvarsY];
```

…and I will be slicing them by unique values of NvarsY. My current best attempt is:

```
vector[(NvarsX +1) * N_ind] theta [NvarsY];
...other stuff...
for (dv in 1:NvarsY){
// pack up betas into thetas
theta = append_row( to_vector(beta0_ind[dv, ]), to_vector( beta_ind[dv, 1:NvarsX, ] )) );
}
```

This is giving me the error:

```
Dimension mismatch in assignment; variable name = theta, num dimensions given = 1; right-hand side dimensions = 0
```

I can’t see whats causing this - any suggestions ? Much appreciated!