groupByNotNull

inline fun <T, K : Any> Iterable<T>.groupByNotNull(keySelector: (T) -> K?): Map<K, List<T>>
inline fun <T, K : Any, V : Any> Iterable<T>.groupByNotNull(keySelector: (T) -> K?, valueTransform: (T) -> V?): Map<K, List<V>>
inline fun <T, K : Any> <Error class: unknown class><T>.groupByNotNull(crossinline keySelector: (T) -> K?): Map<K, List<T>>
inline fun <T, K : Any, V : Any> <Error class: unknown class><T>.groupByNotNull(crossinline keySelector: (T) -> K?, crossinline valueTransform: (T) -> V?): Map<K, List<V>>