Merge two maps with custom reduce function for Kotlin

merge-maps.kt

private fun <K, V> Map<K, V>.mergeReduce(other: Map<K, V>, reduce: (V, V) -> V = { a, b -> b }): Map<K, V> {
    val result = LinkedHashMap<K, V>(this.size() + other.size())
    result.putAll(this)
    other.forEach { e ->
        val existing = result[e.key]

        if (existing == null) {
            result[e.key] = e.value
        }
        else {
            result[e.key] = reduce(e.value, existing)
        }
    }

    return result
}