docs: def do_static_merge(cls, c_source, c_target):
          """By the time we're just folding in clusters, there's no need to maintain
          self.INSTANCES and self.clusters, so we just call this method
          """
          c_target.extend(c_source)
          c_source.parent = c_target.parent
          cls.CLUSTERS.remove(c_source)
          for m in c_source.mentions:
              cls.MENTION_TO_CLUSTER[m] = c_target