docs: def add(self, item):
          """
          Add an item to the work queue.
  
          :param item: The work item to add.  An item may be of any
                       type; however, if it is not hashable, then the
                       work queue must either be initialized with
                       ``unique`` set to ``False``, or a ``key``
                       callable must have been provided.
          """
  
          # Are we to uniquify work items?
          if self._unique:
              key = self._key(item) if self._key else item
  
              # If it already has been added to the queue, do nothing
              if key in self._seen:
                  return
  
              self._seen.add(key)
  
          # Add the item to the queue
          self._work.append(item)
  
          # We'll keep a count of the number of items that have been
          # through the queue
          self._count += 1