docs: public <K> UnsortedGrouping<T> groupBy(KeySelector<T, K> keyExtractor) {
  		TypeInformation<K> keyType = TypeExtractor.getKeySelectorTypes(keyExtractor, getType());
  		return new UnsortedGrouping<>(this, new Keys.SelectorFunctionKeys<>(clean(keyExtractor), getType(), keyType));
  	}