module "set"

function new(list)

	local items = {};

	local set = { items = items };

	function set:add(set, item)

		items[item] = true;

	end

	function set:contains(set, item)

		return items[item]

	end

	function set:items(set)

		return items;

	end

	function set:remove(set, item)

		items[item] = nil;

	end

	function set:add_list(set, list)

		for _, item in ipairs(list) do

			items[item] = true;

		end

	end

	function set:include(set, otherset)

		for item in pairs(otherset) do

			items[item] = true;

		end

	end

	function set:exclude(set, otherset)

		for item in pairs(otherset) do

			items[item] = nil;

		end

	end

	return set;

end

function union(set1, set2)

	local set = new();

	local items = set.items;

	for item in pairs(set1.items) do

		items[item] = true;

	end

	for item in pairs(set2.items) do

		items[item] = true;

	end

	return set;

end

function difference(set1, set2)

	local set = new();

	local items = set.items;

	for item in pairs(set1.items) do

		items[item] = true;

	end

	for item in pairs(set2.items) do

		items[item] = nil;

	end

	return set;

end

return _M;