# Add `v` to the array associated with `k`.
	#
	# If there is no array associated, then create it.
	#
	# For the inverse operation, see `remove_one`.
	#
	# ```
	# var m = new MultiHashMap[String, Char]
	# m.add_one("four", 'i')
	# m.add_one("four", 'i')
	# m.add_one("four", 'i')
	# m.add_one("four", 'i')
	# assert m.has_key("four")
	# assert m["four"] == ['i', 'i', 'i', 'i']
	# ```
	fun add_one(k: K, v: V)
	do
		var x = self.get_or_null(k)
		if x != null then
			x.add(v)
		else
			self[k] = [v]
		end
	end