code: def getdef(self, defname, tag='*'):
          
          if defname.startswith('a:'):
              defname = defname[2:]
          for xsd in self.__xsd_trees:
              xpath = "./%s[@name='%s']" % (tag, defname)
              elements = xsd.xpath(xpath)
              if elements:
                  return elements[0]
          raise KeyError("no definition named '%s' found" % defname)
docs: Return definition element with name *defname*