# Convert expression to univariate polynomial over symbolic ring properly

If I create polynomial like this

```
R.<x> = SR[]
var('a')
f = a*x^3+1
```

I get such list of variables

```
sage: f.variables()
(x,)
```

But how to convert existing expression to univariate polynomial over symbolic ring?

When I do like this

```
var('a')
f = a*x^3+1
P.<x> = SR[]
```

I get following output

```
sage: P(f).variables()
()
sage: SR['x'](f).variables()
()
```

How to convert `f`

to polynomial in a right way with `x`

as variable.