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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 _rgb2l(self, mode):
 """Convert from RGB to monochrome L. """

        self._check_modes(("RGB", "RGBA"))

        kb_ = 0.114
        kr_ = 0.299

        r__ = self.channels[0]
        g__ = self.channels[1]
        b__ = self.channels[2]

        y__ = kr_ * r__ + (1 - kr_ - kb_) * g__ + kb_ * b__

        if self.fill_value is not None:
            self.fill_value = ([rgb2ycbcr(self.fill_value[0],
                                          self.fill_value[1],
                                          self.fill_value[2])[0]] +
                               self.fill_value[3:])

        self.channels = [y__] + self.channels[3:]

        self.mode = mode