# Are all elements of `es` in the same subset?
	#
	#     var s = new DisjointSet[Int]
	#     s.add_all([1,2,3,4,5,6])
	#     s.union_all([1,2,3])
	#     assert not s.all_in_same_subset([2,3,4])
	#     s.union_all([1,4,5])
	#     assert s.all_in_same_subset([2,3,4])
	fun all_in_same_subset(es: Collection[E]): Bool
	do
		if es.is_empty then return true
		var nf = find(es.first)
		for e in es do
			var ne = find(e)
			if ne != nf then return false
		end
		return true
	end