| JoinByFirstGroupElementT, TKey(IEnumerableIGroupingTKey, T) | Takes the first element of each group and joins them into a new IEnumerableT. |
| JoinByFirstGroupElementT, TKey(IEnumerableIGroupingTKey, T, FuncT, Boolean) | Takes the first element that satisfies the specified condition of each group and joins them into a new IEnumerableT. |