def initAlgorithm(self, config):
        """
        Parameter setting.
        """
        self.addParameter(
            QgsProcessingParameterMultipleLayers(
                self.INPUT_LAYERS,
                self.tr('Input Layers'),
                QgsProcessing.TypeVector
            )
        )
        self.addParameter(
            QgsProcessingParameterExpression(
                self.CATEGORY_EXPRESSION,
                self.tr('Expression used to find out the category'),
                defaultValue="regexp_substr(@layer_name ,'([^_]+)')"
            )
        )
        self.addOutput(
            QgsProcessingOutputMultipleLayers(
                self.OUTPUT,
                self.tr('Original reorganized layers')
            )
        )