code: public Mapper<K, V> uniqueKey() {
  		return addConstraint(new Restraint<K, V>() {
  			
  			@Override
  			public void checkKeyValue(K key, V value) {
  				checkArgument(!delegate.get().containsKey(key), "The key: %s is already exist.", key);
  			}
  		});
  	}
docs: Add a unique key constraint to this map
  
  @return