purchase_requisition.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
  <data>
    <record model="ir.ui.view" id="purchase_requisition_form_inherit">
      <field name="name">purchase.requisition.form</field>
      <field name="model">purchase.requisition</field>
      <field name="inherit_id" ref="purchase_requisition.view_purchase_requisition_form"/>
      <field name="arch" type="xml">
        <xpath expr="//button[@string='Request a Quotation']" position="before">
          <button name="auto_rfq_from_suppliers" type="object" string="Generate RFQs from suppliers"
                  attrs="{'invisible': ['|',('line_ids','=',[]),('state','!=','in_progress')]}"
                  />
        </xpath>
      </field>
    </record>
  </data>
</openerp>