'Create a basic tree containing authentication information.
Create a etree Element of type requestType and appends the Authorize.net
credentials (they are always required).
:param str requestType: the type of request to send to Authorize.net
See http://developer.authorize.net/api/reference
for available types.
:return: basic etree Element of the requested type
containing credentials information
:rtype: etree._Element'

def _base_tree(self, requestType):

    root = etree.Element(requestType, xmlns=XMLNS)
    auth = etree.SubElement(root, 'merchantAuthentication')
    etree.SubElement(auth, 'name').text = self.name
    etree.SubElement(auth, 'transactionKey').text = self.transaction_key
    return root
