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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _required_channels(self, seg_numbers, channels, dir_name, pb_dir):
 """ Get the channel numbers to be read from each specified segment, given the channel numbers specified for the entire record. Parameters seg_numbers : list List of segment numbers to read. channels : list The channel indices to read for the whole record. Same one specified by user input. Returns ------- required_channels : list List of lists, containing channel indices to read for each desired segment. """


        # Fixed layout. All channels are the same.
        if self.layout == 'fixed':
            required_channels = [channels] * len(seg_numbers)
        # Variable layout: figure out channels by matching record names
        else:
            required_channels = []
            # The overall layout signal names
            l_sig_names = self.segments[0].sig_name
            # The wanted signals
            w_sig_names = [l_sig_names[c] for c in channels]

            # For each segment
            for i in range(len(seg_numbers)):
                # Skip empty segments
                if self.seg_name[seg_numbers[i]] == '~':
                    required_channels.append([])
                else:
                    # Get the signal names of the current segment
                    s_sig_names = rdheader(
                        os.path.join(dir_name, self.seg_name[seg_numbers[i]]),
                        pb_dir=pb_dir).sig_name
                    required_channels.append(_get_wanted_channels(
                        w_sig_names, s_sig_names))

        return required_channels