code: func (w *Watch) addInst(inst *Instance) (known, identical bool) {
  	var i *Instance
  	i, known = w.instances[inst.ID]
  	identical = known && i.Equal(inst)
  	w.instances[inst.ID] = inst
  	return
  }
docs: // addInst adds the given instance to the local state and returns whether the
  // instance already existed in the local state, and if it did exist, whether it
  // was identical.