Xpath selection not working in Odoo 9

Question

In Odoo 9, I'm trying to inherit the purchase order form view (file: addons/purchase/purchase_view.xml, record: <record id="purchase_order_form" model="ir.ui.view">). What I need to do is, just edit an attribute of one page tag. I tried to select the desired page by using xpath, but it's not working fine.

Parent View Structure

<notebook>
    <page string="Products">
        ......
        ......
        <notebook>
        <page string="Notes">
            <field name="name"/>
        </page><page string="Invoices and Incoming Shipments">
            <field name="invoice_lines"/>
            <field name="move_ids"/>
        </page>
        </notebook>
        ......
        ......
    </page>
    <page string="Deliveries &amp; Invoices">


    </page>
</notebook>

In my view file, I used the following code to select the <page string="Deliveries &amp; Invoices"> and this is not working as expected.

<record model="ir.ui.view" id="purchase_order_type_form_view_inherit">
    <field name="name">purchase.order.form.inherit</field>
    <field name="model">purchase.order</field>
    <field name="inherit_id" ref="purchase.purchase_order_form"/>
    <field name="priority">10000</field>
    <field name="arch" type="xml">
        <xpath expr="//notebook/page[2]" position="attributes">
            <attribute name="groups">custom_module.manager</attribute>
        </xpath>
    </field>
</record>

Thanks in advance.

Answer 1

<record model="ir.ui.view" id="purchase_order_type_form_view_inherit">
        <field name="name">purchase.order.form.inherit</field>
        <field name="model">hr_employee</field>
        <field name="inherit_id" ref="purchase.purchase_order_form"/>
        <field name="priority">10000</field>
        <field name="arch" type="xml">
            <xpath expr="//form/sheet/notebook/" position="after">
                <page>
                    <group>
                            <group colspan="4" col="4">

                                <label for="employment_ids" colspan="4" />
                                <field name="employment_ids" colspan="4" nolabel="1">
                                    <form string="Employee Employment">
                                        <field name="organization" />
                                        <field name="job_title" />
                                        <field name="entered_date" />
                                        <field name="resigned_date" />
                                        <field name="resigned_reason" />
                                        <field name="responsibility" />
                                        <field name="wage" />
                                    </form>
                                    <tree string="Employee Employment" editable="bottom">
                                        <field name="organization" />
                                        <field name="job_title" />
                                        <field name="entered_date" />
                                        <field name="resigned_date" />
                                        <field name="resigned_reason" />
                                        <field name="responsibility" />
                                        <field name="wage" />
                                    </tree>
                                </field>
                            </group>
                    </group>
                </page>
            </xpath>
        </field>
    </record>