docs: def getGUA(self, filterByPrefix=None):
          """get expected global unicast IPv6 address of OpenThreadWpan
  
          Args:
              filterByPrefix: a given expected global IPv6 prefix to be matched
  
          Returns:
              a global IPv6 address
          """
          print '%s call getGUA' % self.port
          print filterByPrefix
          globalAddrs = []
          try:
              # get global addrs set if multiple
              globalAddrs = self.getGlobal()
  
              if filterByPrefix is None:
                  return self.__padIp6Addr(globalAddrs[0])
              else:
                  for line in globalAddrs:
                      line = self.__padIp6Addr(line)
                      print "Padded IPv6 Address:" + line
                      if line.startswith(filterByPrefix):
                          return line
                  print 'no global address matched'
                  return str(globalAddrs[0])
          except Exception, e:
              ModuleHelper.WriteIntoDebugLogger('getGUA() Error: ' + str(e))
              return e