# Return a view of `e` in the poset.
	# This allows to view the elements in their relation with others elements.
	#
	#     var poset = new POSet[String]
	#     poset.add_chain(["A", "B", "D"])
	#     poset.add_chain(["A", "C", "D"])
	#     var a = poset["A"]
	#     assert a.direct_greaters.has_exactly(["B", "C"])
	#     assert a.greaters.has_exactly(["A", "B", "C", "D"])
	#     assert a.direct_smallers.is_empty
	#
	# REQUIRE: has(e)
	fun [](e: E): POSetElement[E]
	do
		assert elements.keys.has(e)
		return self.elements[e]
	end