<record id="product_template_form_view" model="ir.ui.view">
<field name="name">product.template.form.inherit</field>
<field name="model">product.template</field>
<field name="inherit_id" ref="product.product_template_form_view"/>
<field name="arch" type="xml">
<xpath expr="//page[@name='sales']" position="attributes">
<attribute name="invisible">0</attribute>
</xpath>
<xpath expr="//page[@name='sales']" position="after">
<page name="pos" string="Point of Sale" attrs="{'invisible': [('sale_ok','=',False)]}">
<group name="pos">
<group>
<field name="available_in_pos"/>
<field name="pos_categ_id" attrs="{'invisible': [('available_in_pos', '=', False)]}" string="Category"/>
<field name="to_weight" attrs="{'invisible': [('available_in_pos', '=', False)]}"/>
</group>
</group>
</page>
</xpath>
</field>
</record>

how to add my field after  '' to_weight '' ?
​

i try this but doesn't work 

<xpath expr="//page[@name='sales']" position="inside">
<xpath expr="//group[@name='pos']" position="inside">
  <group>
      <field name="my_field" attrs="{'invisible': [('available_in_pos', '=', False)]}"/>
  </group>
</xpath>
            </xpath>

​