code: def collect_all_bucket_keys(self):
          
          if len(self.childs) == 0:
              # This is a leaf so just return the bucket key (we reached the bucket leaf)
              #print 'Returning (collect) leaf bucket key %s with %d vectors' % (self.bucket_key, self.vector_count)
              return [self.bucket_key]
  
          # Not leaf, return results of childs
          result = []
          for child in self.childs.values():
              result = result + child.collect_all_bucket_keys()
  
          return result
docs: Just collects all buckets keys from subtree