# Is there a direct edge from `f` to `t`?
	#
	# ~~~
	# var pos = new POSet[String]
	# pos.add_chain(["A", "B", "C"]) # add A->B->C
	# assert pos.has_direct_edge("A", "B") == true
	# assert pos.has_direct_edge("A", "C") == false
	# assert pos.has_edge("A", "C")        == true
	# ~~~
	#
	# Note that because of loops, the result may not be the expected one.
	fun has_direct_edge(f,t: E): Bool
	do
		if not elements.keys.has(f) then return false
		var fe = self.elements[f]
		return fe.dtos.has(t)
	end