### Matrix with polynomial entries in Sage.

I am trying to define a function to produce matrices with polynomial entries. I defined the following

```
def BraidMatrices(i,n,z):
r=identity_matrix(n,n)
r[i,i]=0
r[i,i+1]=1
r[i+1,i]=1
r[i+1,i+1]=z
return r
z=VariableGenerator('z')
n=3
t1=BraidMatrices(1,n,z[1])
t1
```

But it has some errors:
1099 n = self.pyobject()
1100 except TypeError:
-> 1101 raise TypeError("unable to convert %r to an integer" % self)
1102 if isinstance(n, sage.rings.integer.Integer):
1103 return n

TypeError: unable to convert z1 to an integer

How to define a matrix with polynomial entries in Sage? Thank you very much.