I try to add a char field in Purchase Order form view after the Total field field[@name='amount_total']. I've been struggle with this, please help me. Right now, the error shows invalid XML architecture. Here's my code:

__init__.py

import po

__openerp__.py

{
    'name': 'PO money in text',
    'version': '1.0',
    'author': 'GonG',
    'category': 'Tools',
    'depends': ['purchase'],
    'data': ['po.xml'],
    'demo': [],
    'installable': True,
}

po.py from openerp.osv import fields, osv

class PO(osv.osv):

    _inherit = 'purchase.order'
    _columns = {
        'money_in_text': fields.char('Money in Word', size=200),
    }

PR()

po.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>
        <record model="ir.ui.view" id="PO">
            <field name="name">purchase.order.form</field>
            <field name="model">purchase.order</field>
            <field name="inherit_id" ref="purchase.purchase_order_form" />
            <field name="arch" type="xml">
                <xpath expr="/form/sheet/notebook/page[@string='Purchase Order']/group/field[@name='amount_total']" position="after">
                    <field name="money_in_text" />
                </xpath>
            </field>
        </record>
    </data>
</openerp>

Thank you very much