I define a 3+1-dimensional manifold **M**:

```
n = 3
M = Manifold(1+n, 'M', structure='Lorentzian')
```

and a chart **X** with spherical coordinates **t**, **r**, **th** and **ph**.

```
X.<t,r,th,ph> = M.chart('t r:(0,+oo) th:(0,pi) ph:(0,2*pi)')
```

I define the following symbolic function **Ne**, which depends on **r** and a new variable, **eps**:

```
eps = var('eps')
Ne = function('Ne')(r,eps)
```

Now, I define a vector field, **u**, as:

```
u = M.vector_field('u')
u[0] = 2*Ne; u[1] = Ne; u[2] = 0; u[3] = 0
```

At this point, I want to differentiate each term of **u** with respect to **eps**. However, when I write:

```
diff(u[0],eps)
```

I get:

ValueError: tuple.index(x): x not in tuple

However, I would expect to obtain:

2*diff(Ne(r, eps), eps)

I guess this is because I am differentiating a vector field from the manifold with respect to a variable (eps) which is not on the chart. In fact, if I differentiate with respect to $r$, then everything goes right. Is there a way I can fix this?