# Get the first item.
	# Is equivalent with `self[0]`.
	#
	#     var a = [1,2,3]
	#     assert a.first   == 1
	#
	# REQUIRE `not is_empty`
	redef fun first
	do
		assert not_empty: not is_empty
		return self[0]
	end

	redef var first

	redef fun first
	do
		mutex.lock
		var r = real_collection.first
		mutex.unlock
		return r
	end

	redef fun first do return item

	redef fun first do return self.map._items.first.first

	redef fun first do return self.map._items.first.second

	redef fun first do return self.map._first_item._key

	redef fun first do return self.map._first_item._value

	#     var tree = new OrderedTree[Int]
	#     tree.add(null, 1)
	#     tree.add(1, 11)
	#     assert tree.first == 1
	redef fun first do return roots.first

	# `first` is made an alias of `peek` to avoid bad surprises
	redef fun first do return peek

	redef fun first
	do
		assert _array.length > 0
		return _array.first
	end

	# O(1)
	redef fun first do return _head.as(not null).item

	redef fun first: Int
	do
		assert _length > 0
		return _values[0]
	end

	redef fun first
	do
		assert _the_length > 0
		return _first_item._key
	end