Inheritence of a tree inside a form

Hello all,

I'm trying to add a field into a tree included in a form. The original form has a structure like this :

...
<form string=
    <notebook>
        <page>
            <field colspan="4" mode="tree,form,graph" name="order_line" widget="one2manylist">
            <form string="Sale Order Line">
             .....
            </form>
            <tree string="Sales order lines">
            .....
            </tree>
      </field>
....

So, I wanted to add some fields inside the form 'form_2' and the tree 'tree_2'

For the form, no problem it's working fine. But for the tree, impossible to add some fields. If,before declaring the new fiels I add the tag <tree string="Sales order lines">, this is the graph that is displayed ! if I add nothing, the fiels are coming in my form and not in my tree.

I dont know which tag to add to add fields in my tree...

Thanks very much for your help