### Non-homogenous Ideals In Unital Associative Free Algebras

~~Hi all,~~

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

in ~~an ~~a unital associative free algebra ~~K.<x,y>. But, the error I have faced to is the following:~~`K.<x,y>`

.

~~ArithmaticError: ~~But I get:

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

Is there any way to define such an ideal in ~~sage.~~

ThanksSage?