purchase_order.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>

        <record id="purchase_order_form" model="ir.ui.view">
            <field name="name">purchase.order.form</field>
            <field name="model">purchase.order</field>
            <field name="inherit_id" ref="purchase.purchase_order_form"></field>
            <field name="arch" type="xml">
                <button name="action_cancel_draft" position="after" >
                    <button name="new_revision" string="New revision" type="object" attrs="{'invisible': ['|',('state', '!=', 'cancel'),('current_revision_id','!=', False)]}"/>
                </button>
                <button name="action_cancel_draft" position="attributes" >
                    <attribute name="invisible">1</attribute>
                </button>
                <notebook position="inside">
                    <page string="Revisions">
                        <field name="old_revision_ids"></field>
                        <group attrs="{'invisible': [('active', '=', True)]}">
                            <field name="current_revision_id"></field>
                            <field name="active" invisible="1"/>
                        </group>
                    </page>
                </notebook>
            </field>
        </record>
    </data>
</openerp>