code: func RequireKeyUnchanged(key string) PreconditionFunc {
  	return func(patch interface{}) bool {
  		patchMap, ok := patch.(map[string]interface{})
  		if !ok {
  			return true
  		}
  
  		// The presence of key means that its value has been changed, so the test fails.
  		_, ok = patchMap[key]
  		return !ok
  	}
  }
docs: // RequireKeyUnchanged returns a precondition function that fails if the provided key
  // is present in the patch (indicating that its value has changed).