<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _to_p(self, mode):
 """Convert the image to P or PA mode. """


        if self.mode.endswith("A"):
            chans = self.channels[:-1]
            alpha = self.channels[-1]
            self._secondary_mode = self.mode[:-1]
        else:
            chans = self.channels
            alpha = None
            self._secondary_mode = self.mode

        palette = []
        selfmask = chans[0].mask
        for chn in chans[1:]:
            selfmask = np.ma.mask_or(selfmask, chn.mask)
        new_chn = np.ma.zeros(self.shape, dtype=int)
        color_nb = 0

        for i in range(self.height):
            for j in range(self.width):
                current_col = tuple([chn[i, j] for chn in chans])
                try:
                    next(idx
                         for idx in range(len(palette))
                         if palette[idx] == current_col)
                except StopIteration:
                    idx = color_nb
                    palette.append(current_col)
                    color_nb = color_nb + 1

                new_chn[i, j] = idx

        if self.fill_value is not None:
            if self.mode.endswith("A"):
                current_col = tuple(self.fill_value[:-1])
                fill_alpha = [self.fill_value[-1]]
            else:
                current_col = tuple(self.fill_value)
                fill_alpha = []
            try:
                next(idx
                     for idx in range(len(palette))
                     if palette[idx] == current_col)
            except StopIteration:
                idx = color_nb
                palette.append(current_col)
                color_nb = color_nb + 1

            self.fill_value = [idx] + fill_alpha

        new_chn.mask = selfmask
        self.palette = palette
        if alpha is None:
            self.channels = [new_chn]
        else:
            self.channels = [new_chn, alpha]
        self.mode = mode