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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 layer_keyword(self, keyword, value):
 """Get a layer according to a keyword and its value. :param keyword: The keyword to check. :type keyword: basestring :param value: The value to check for the specific keyword. :type value: basestring :return: The QGIS layer. :rtype: QgsMapLayer .. versionadded:: 4.0 """

        for layer in sorted(self.layers(), reverse=True):
            qgis_layer = self.layer(layer)
            if qgis_layer.keywords.get(keyword) == value:
                return qgis_layer
        return None