### Using matrix elements as arguments

I have a rather easy question, or so it would seem. I have looked for an answer but was unable to find one anywhere so I'm asking it here.

I am making a very simple iterative algorithm for which the input as well as the output at the end of every iteration is a vector (or matrix for that matter). What I want to do is use these elements as arguments for several functions during each of the iteration. So for example

```
x=var('x')
y=var('y')
z=matrix(2,1,[ [1],[1] ]
f=x^2+y^3
H=f.hessian()
```

Then what I would like to do is say

```
H(z[0],z[1])
```

or

```
H(z)
```

But no matter what I try I can't seem to get it to work. Ideas?