You can make a tensor with `ufl.as_tensor`

that is a mixture of `ufl.SpatialCoordinate`

and constants (say for time)`

For instance you could do something like

```
x = ufl.SpatialCoordinate(domain)
a = dolfinx.fem.Constant(domain, 0.)
t = dolfinx.fem.Constant(domain, 0.1)
D = ufl.as_tensor(((a*x[0]-t, 0),(x[1], t)))
```

and update a and t in your time loop.