def icons(self, left=None):
        """
        Return the ICONS with left variable *left*, or all ICONS.

        Args:
            left: the left variable of the ICONS to return; if `None`,
                return all ICONS
        """
        if left is not None:
            return self._icons[left]
        else:
            return list(chain.from_iterable(self._icons.values()))