Returns a :class:`Counter` object that counts the number of tasks with
        given status (use the string representation of the status as key).

def status_counter(self):
        """
        Returns a :class:`Counter` object that counts the number of tasks with
        given status (use the string representation of the status as key).
        """
        # Count the number of tasks with given status in each work.
        counter = self[0].status_counter
        for work in self[1:]:
            counter += work.status_counter

        return counter