# set_name causes an error on external_derivative display

I have the following code:

```
R3 = Manifold(3, 'R3', start_index=1, latex_name=r'\mathbb{R}^3')
cartesian3d.<x,y,z> = R3.chart()
omega = R3.diff_form(1, 'omega', latex_name=r'\omega')
omega[:] = (x^2+y*z+e^y, y^3+x*z+x*e^y+z^2, e^z+x*y+2*y*z)
print omega.category()
omega.display()
domega = omega.exterior_derivative()
domega.set_name(domega, r'\mathrm{d}\omega')
print domega.category()
domega.display()
```

I get a **TypeError: unsupported operand type(s) for +: 'DiffFormFreeModule_with_category.element_class' and 'str'**

If I delete the **set_name** command the code works fine.

Daniel