Quotient Ideals In Unital Associative Free Algebras

[Abdolrasool Bahari-fard]

Hi all,

I created an unital associative free algebra 

sage: F.<x,y>=FreeAlgebra(QQ, implementation='letterplace').

But for having a twisted ideal 

I=F*[x*y*x*y-x*y]*F in F, I have faced the following error:

ArithmaticError : can only subtract the elements of the same degree.

Am I in wrong place in sage?

Also I want to define an infinite dimensional quotient algebra in non-commutative algebra F/I.

Is there a way to define that in sage or other part of sage like Gp or Singular?

Thanks