code: def _validate(self, writing=False):
          
          # channel type and association must be specified.
          if not ((len(self.index) == len(self.channel_type)) and
                  (len(self.channel_type) == len(self.association))):
              msg = ("The length of the index ({index}), channel_type "
                     "({channel_type}), and association ({association}) inputs "
                     "must be the same.")
              msg = msg.format(index=len(self.index),
                               channel_type=len(self.channel_type),
                               association=len(self.association))
              self._dispatch_validation_error(msg, writing=writing)
  
          # channel types must be one of 0, 1, 2, 65535
          if any(x not in [0, 1, 2, 65535] for x in self.channel_type):
              msg = ("channel_type specified as {channel_type}, but all values "
                     "must be in the set of\n\n"
                     "    0     - colour image data for associated color\n"
                     "    1     - opacity\n"
                     "    2     - premultiplied opacity\n"
                     "    65535 - unspecified\n")
              msg = msg.format(channel_type=self.channel_type)
              self._dispatch_validation_error(msg, writing=writing)
docs: Verify that the box obeys the specifications.