# Pullback from a transition_map

Hi All,

I seem to be unable to pullback a differential form from a transition_map, although the transition_map has all the information to calculate the pullback:

```
sage: Space = Manifold(3, 'Space', start_index=1)
sage: cartesian3d.<x, y, z> = Space.chart()
sage: spherical.<rho, theta, phi> = Space.chart(r'rho:(0,+oo):\rho theta:(0,pi):\theta phi:(0,2*pi):\phi')
sage: C2S = spherical.transition_map(cartesian3d, [rho*sin(theta)*cos(phi), rho*sin(theta)*sin(phi), rho*cos(theta)])
sage: tau = Space.diff_form(3, 'tau', latex_name=r'\tau')
sage: tau[1,2,3] = 1
sage: Ptau = C2S.pullback(tau)
```

AttributeError: 'DiffCoordChange' object has no attribute 'pullback'

Why is that?

Is there any way to coerce a transition_map to become a diff_map?

Thanks,

Daniel