Comments on delete action

All,

I still haven't regained my access to the wiki, but I was checking section
"11.4 The delete Element" [1] and found a few things to discuss.

1. "The delete action is terminated with no effect if the delete context is
the empty sequence."

I am not sure we need this restriction. The evaluation context item is
present in order to allow for `ref` to evaluate. But some expressions don't
require a context, for example this would make sense:

    <delete
        context="()"
        ref="instance('my-instance')/bar"/>

2. "The behavior of the delete action is undefined if the Sequence Binding
node-sequence contains nodes from more than one instance"

I am not sure why we need this to be undefined: the action should just
delete nodes from all instances in question.

    <delete
        ref="instance('my-instance')/bar, instance('your-instance')/baz"/>

3. "Otherwise, the Sequence Binding is not expressed, so the Sequence
Binding node-sequence is set equal to the delete context node with a
position and size of 1."

I am not sure why we need to allow <xf:delete/> or <xf:delete
context="foo"/>. We should require `bind` or `ref`. I hadn't even realized
that this was possible.

We don't have any examples in the spec without a `ref` or `bind` attribute.

4. cannot delete "namespace node"?

I am not sure about this one, but there might be cases where this is ok.

5. We must add what to do if the sequence points to some atomic values. We
should say that those are ignored just as readonly items are.

-Erik

[1] https://www.w3.org/MarkUp/Forms/wiki/XForms_2.0#The_delete_Element