# Non-homogenous Ideals In Unital Associative Free Algebras

I want to define a two-sided ideal `I = [x*y*x*y - x*y]`

in a unital associative free algebra `K.<x,y>`

.

But I get:

```
ArithmeticError: Can only subtract elements of the same degree.
```

Is there any way to define such an ideal in Sage?