Again on symbolic computation

I said that symbolic computation was ok, but making more and more examples
I found some problem.

I refer to

http://wiki.axiom-developer.org/SandBoxNoncommutativePolynomials

with notations

Q=FRAC INT
K=Q(a,b)
H=K[x,y]


Now I use relations of degree 3 for x and y, namely:

yxy=xyx
x^3=ax^2+bx+c
y^3=ax^2+by+c

Accordingly, I modified the core function subs to

subs(w:M):H ==
   -- y*x*y=x*y*x
   n:=div(w,(y::V*x::V*y::V)$M)$M
   n case ok => monom(n.ok.lm,1)$H * (x::V*y::V*x::V)$H * monom(n.ok.rm,1)$H
   -- x*x*x=a*x*x+b*x+c
   n:=div(w,(x::V^3)$M)$M
   n case ok => monom(n.ok.lm,1)$H * (a::K*(x::V)^2+b::K*x::V+c::K)$H * monom(n.ok.rm,1)$H
   -- y*y*y=a*y*y+b*y+c
   n:=div(w,(y::V^3)$M)$M
   n case ok => monom(n.ok.lm,1)$H * (a::K*y::V*y::V+b::K*y::V+c::K)$H * monom(n.ok.rm,1)$H
   -- no change
   monom(w,1)$H

and I use the auxilliary function

reduce(p:H):H ==
   p2 := newpoly(p)
   p3 := newpoly(p2)
   while p3 ~= p2 repeat
    p2 := p3
    p3 := newpoly(p2)
   p3

The problem is that not every monomial gets reduced to its "lowest degree"
form. Take, for example,
w=xxyxxyx

you get

reduce(w)=w

and this happens because it doesn't contain any yxy,xxx,yyy. On the other
hand, it DOES reduce:

xxyxxyx
     ---
xxyxyxy
   ---
xxxyxxy

and so now a x^3 appears, which can be reduced.

Adding the relation for xyx=yxy doesn't help: you go in loop in many
cases. So, how to handle this kind of problem?


Thanks again

Fabio

PS: I tried to add this to the relative sandbox page, but I have not been 
able: I added the "reduce" function, but it complains that H is not a 
valid type. That is, it doesn't load automatically all the previous 
definition. How do I get this?