code: func (m *Memory) removeExpired() int {
  	removed := 0
  	now := m.Clock().Now().UTC()
  	for {
  		if len(*m.heap) == 0 {
  			break
  		}
  		item := m.heap.PeekEl()
  		if now.Before(item.Expires) {
  			break
  		}
  		m.heap.PopEl()
  		m.tree.Delete(item)
  		m.Debugf("Removed expired %v %v item.", string(item.Key), item.Expires)
  		removed++
  	}
  	if removed > 0 {
  		m.Debugf("Removed %v expired items.", removed)
  	}
  	return removed
  }
docs: // removeExpired makes a pass through map and removes expired elements
  // returns the number of expired elements removed