code: def as_xml(self):
          
          # Build tree
          spacing = ("\n" if (self.pretty_xml) else "")
          root = Element('error', {'xmlns': I3F_NS})
          root.text = spacing
          e_parameter = Element('parameter', {})
          e_parameter.text = self.parameter
          e_parameter.tail = spacing
          root.append(e_parameter)
          if (self.text):
              e_text = Element('text', {})
              e_text.text = self.text
              e_text.tail = spacing
              root.append(e_text)
          # Write out as XML document to return
          tree = ElementTree(root)
          xml_buf = io.BytesIO()
          if (sys.version_info < (2, 7)):
              tree.write(xml_buf, encoding='UTF-8')
          else:
              tree.write(xml_buf, encoding='UTF-8',
                         xml_declaration=True, method='xml')
          return(xml_buf.getvalue().decode('utf-8'))
docs: XML representation of the error to be used in HTTP response.
  
          This XML format follows the IIIF Image API v1.0 specification,
          see <http://iiif.io/api/image/1.0/#error>