# astropy__astropy-12907
# Loki Mode Multi-Agent Patch
# Attempts: 1

Based on the architect's analysis, I need to generate a patch for the astropy repository's `separable.py` file. The issue is in the `_cstack` function where it uses `np.eye(right.n_inputs)` instead of recursively computing the separability matrix for nested CompoundModels.

Let me generate the patch based on the analysis:

```
--- a/astropy/modeling/separable.py
+++ b/astropy/modeling/separable.py
@@ -242,7 +242,11 @@ def _cstack(left, right):
         cright = _coord_matrix(right, 'right', noutp)
     else:
         cright = np.zeros((noutp, right.n_inputs))
-        cright[-right.n_outputs:, -right.n_inputs:] = np.eye(right.n_inputs)
+        if isinstance(right, CompoundModel):
+            cright[-right.n_outputs:, -right.n_inputs:] = _separable(right)
+        else:
+            cright[-right.n_outputs:, -right.n_inputs:] = np.eye(right.n_inputs)
+
     return np.vstack([cleft, cright])
```