# A problem on Groebner Fans with more than 10 variables

```
sage: X=[var('x%d' % i) for i in range(20)]
```

Up to 10 variables, everything is alright:

```
sage: R=PolynomialRing(QQ,"x",10)
sage: R.ideal(x0).groebner_fan()
Groebner fan of the ideal:
Ideal (x0) of Multivariate Polynomial Ring in x0, x1, x2, x3, x4, x5, x6, x7, x8, x9 over Rational Field
```

But with at least one more variable, there is a problem:

```
sage: R=PolynomialRing(QQ,"x",11)
sage: R.ideal(x0).groebner_fan()
---------------------------------------------------------------------------
RuntimeError Traceback (most recent call last)
<ipython-input-49-29624ce56396> in <module>()
----> 1 R.ideal(x0).groebner_fan()
/home/sebastien/sage/local/lib/python2.7/site-packages/sage/rings/polynomial/multi_polynomial_ideal.pyc in groebner_fan(self, is_groebner_basis, symmetry, verbose)
3486 import sage.rings.polynomial.groebner_fan as groebner_fan
3487 return groebner_fan.GroebnerFan(self, is_groebner_basis=is_groebner_basis,
-> 3488 symmetry=symmetry, verbose=verbose)
3489
3490 @cached_method
/home/sebastien/sage/local/lib/python2.7/site-packages/sage/rings/polynomial/groebner_fan.pyc in __init__(self, I, is_groebner_basis, symmetry, verbose)
810 raise TypeError, "I must be a multivariate polynomial ideal"
811 if prefix_check([str(R_gen) for R_gen in I.ring().gens()]) != True:
--> 812 raise RuntimeError, "Ring variables cannot contain each other as prefixes"
813 S = I.ring()
814 R = S.base_ring()
RuntimeError: Ring variables cannot contain each other as prefixes
```

What's the problem ? How have more than 10 variables ?

I view this as a bug - see http://trac.sagemath.org/ticket/15644