def clear(self):
        """Clear current state."""
        self.exposures = []
        self.exposure_labels = []
        self.exposure_combo_boxes = []
        self.exposure_edit_buttons = []
        self.mode = CHOOSE_MODE

        self.layer_purpose = None
        self.layer_mode = None
        self.special_case_index = None

        self.value_maps = {}
        self.thresholds = {}

        # Temporary attributes
        self.threshold_classes = OrderedDict()
        self.active_exposure = None

        self.list_unique_values = None
        self.tree_mapping_widget = None

        clear_layout(self.left_layout)
        clear_layout(self.right_layout)
Clear current state.