~~Polinomial: ~~Polynomial: find the common factor

Hello, assuming I have a ~~polinomial ~~polynomial like this:

```
sage: var('y, x')
(y, x)
sage: y = x^2 + 6*x^3 + 3*x^4
```

When I use collect_common_factors(), I get:

```
(3*x^2 + 6*x + 1)*x^2
```

Now, is there a way to know which are the common_factors collected?

In this case, I'm expecting this output:

```
x^2
```

Thank you everyone!