# Remove an occurrence of `v` from the array associated with `k`.
	#
	# If the associated array does not contain `v`, do nothing. If the
	# associated array only contain one element and this element is `v`, remove
	# the key `k`.
	#
	# In a nutshell, does the inverse operation of `add_one`.
	#
	# ```
	# var m = new MultiHashMap[String, Char]
	# m["four"]        =  ['4', 'i', 'i', 'i', 'i']
	# m.remove_one("four", 'i')
	# assert m["four"] == ['4', 'i', 'i', 'i']
	#
	# m = new MultiHashMap[String, Char]
	# m.add_one("one", '1')
	# m.remove_one("one", '?')
	# assert m["one"] == ['1']
	# m.remove_one("one", '1')
	# assert not m.has_key("one")
	# assert m["one"] == new Array[Char]
	#
	# m = new MultiHashMap[String, Char]
	# m.add_one("one", '1')
	# m.remove_one("two", '2')
	# assert not m.has_key("two")
	# assert m["one"] == ['1']
	# assert m["two"] == new Array[Char]
	# ```
	fun remove_one(k: K, v: V)
	do
		var x = get_or_null(k)
		if x != null then
			x.remove(v)
			if x.is_empty then keys.remove(k)
		end
	end