Magento - How to add button to order view page in admin

 - How could we add button to the top of order view page in admin?   (behind back, edit, cancel, hold, invoice, ship).

-  We will either have to override the mage block or edit the core.  But to add you can do something like this:

app/code/core/Mage/Adminhtml/Block/Sales/Order/View.php

Mage_Adminhtml_Block_Sales_Order_View

Add the following to the constructor:

 $this->_addButton('order_reorder', array(
                        'label'     => Mage::helper('sales')->__('Print'),
                        'onclick'   => 'setLocation(\'' . $this->getUrl('ssireports/index/invoice') . '\')',
        ));        ));