### Multivariable taylor series with relationship between variables

Let's consider a function `f(x,y,z)`

and the following relationship between the variables:

```
f(x,y,z) = x+y+z
x << y << z around 0
```

I would like the first order Taylor expansion of f around 0. And I would expect this result:

```
>> taylor(f, (x,0),(y,0),(z,0),1)
z
```

But I have :

```
>> taylor(f, (x,0),(y,0),(z,0),1)
x+y+z
```

How can I do it?