Add to the current progress amount

        Add `delta` to the current progress amount. This also updates
        :attr:`rate` and :attr:`remaining_time`.

        The :attr:`current` progress is never less than 0 or greater
        than :attr:`done`.

        :param delta: amount to add, may be negative
        :param done: new value to use for done

def add_progress(self, delta, done=None):
        """Add to the current progress amount

        Add `delta` to the current progress amount. This also updates
        :attr:`rate` and :attr:`remaining_time`.

        The :attr:`current` progress is never less than 0 or greater
        than :attr:`done`.

        :param delta: amount to add, may be negative
        :param done: new value to use for done
        """
        if done is not None:
            self.done = done
        self.bar.current = max(min(self.done, self.current + delta), 0)
        self.rate_display.set_text(self.rate_text)
        self.remaining_time_display.set_text(self.remaining_time_text)
        return self.current == self.done