### Constant coefficient of symbolic expression

Suppose we have the following

```
sage: var("x,y,z")
sage: expr = x*y+z^2+4
```

I am looking for a function which does something like

```
sage: expr.constant_coefficient()
4
```

However, I did not find such a function. If I use `coefficient`

I can get the desired result using

```
sage: expr.coefficient(x,0).coefficient(y,0).coefficient(z,0)
4
```

Of course if I have more variables that gets more tedious and I'd write a small helper function which goes through the variables contained in `expr`

. I feel like this is much too complicated and I'm just overlooking some function. Is anyone aware of some cleaner way to do this if the expression contains a lot of variables?

Thank you for your help!