# The parent of the element `e`
	#
	# Roots have `null` as parent.
	#
	# ~~~
	# var tree = new OrderedTree[Int]
	# tree.add(1, 2)
	# assert tree.parent(2) == 1
	# assert tree.parent(1) == null
	# ~~~
	fun parent(e: E): nullable E do return parents[e]