def pop_task(self, number):
        """
        Removes a previously added **task** from the ``NuMap`` instance.
        
        Arguments:
        
          - number (``int`` or ``True``) A positive integer specifying the 
            number of **tasks** to pop. If  number is set ``True`` all **tasks**
            will be popped.
        
        """
        if not self._started.isSet():
            if number is True:
                self._tasks = []
                self._tasks_tracked = {}
            elif number > 0:
                last_task_id = len(self._tasks) - 1
                for i in xrange(number):
                    self._tasks.pop()
                    self._tasks_tracked.pop(last_task_id - i, None)
        else:
            log.error('%s cannot delete tasks (is started).' % self)
            raise RuntimeError('%s cannot delete tasks (is started).' % self)