Ignore fields and class types

It would be very helpful if you could configure XStream to ignore certain fields and certain classes when outputing xml.

For example

public class X {
private int v;
}

public class Y {
private int w;
private int z;
private X x;
}

would generally create something like:

<Y>
<w>0</w>
<z>0</z>
<x>
<v>0</v>
</x>
</Y>

putting in the following:

xstream.ignoreClass(X.class);
xstream.ignoreField(Y.class, "z");

would yield:

<Y>
<w>0</w>
</Y>