code: def append_instances(cls, inst1, inst2):
          
          msg = inst1.equal_headers(inst2)
          if msg is not None:
              raise Exception("Cannot appent instances: " + msg)
          result = cls.copy_instances(inst1)
          for i in xrange(inst2.num_instances):
              result.add_instance(inst2.get_instance(i))
          return result
docs: Merges the two datasets (one-after-the-other). Throws an exception if the datasets aren't compatible.
  
          :param inst1: the first dataset
          :type inst1: Instances
          :param inst2: the first dataset
          :type inst2: Instances
          :return: the combined dataset
          :rtype: Instances