Just checking - I don't think this exists in the stdlib?

fun <K, V> Iterable<Pair<K, V>>.groupByKey(): Map<K, List<V>> = this.groupBy({ it.first }, { it.second})

Is it a bad idea?

if we had something like a

interface Multimap<K, V> : Map<K, List<V>>

then this method would could naturally be called

.toMultimap()