# Straight line parametrization with diff_map

I have the following code:

```
R2 = Manifold(2, 'R2', latex_name=r'\mathbb{R}^2')
cartesian2d.<x,y> = R2.chart()
R1 = Manifold(1, 'R1', start_index=1, latex_name=r'\mathbb{R}')
cartesian1d.<t> = R1.chart()
A = R2.point((3,2), chart=cartesian2d, name='A')
B = R2.point((-1,2), chart=cartesian2d, name='B')
C_1 = R1.diff_map(R2, t*B.coord()+(1-t)*A.coord())
```

The last line issues an error **TypeError: unable to convert t to an integer**.

I want to define a parametrization of a straight line from point A at `t=0`

to point B at `t=1`

. How to do that?

Daniel