docs: def estimate_reduce(interface, state, label, inp):
      """Estimate the cluster centers for each cluster."""
      centers = {}
      for i, c in inp:
          centers[i] = c if i not in centers else state['update'](centers[i], c)
  
      out = interface.output(0)
      for i, c in centers.items():
          out.add(i, state['finalize'](c))