# Is there an edge (transitive or not) from `f` to `t`?
	#
	# SEE: `add_edge`
	#
	# Since the POSet is reflexive, true is returned if `f == t`.
	#
	# ~~~
	# var pos = new POSet[String]
	# pos.add_node("A")
	# assert pos.has_edge("A", "A") == true
	# ~~~
	fun has_edge(f,t: E): Bool
	do
		if not elements.keys.has(f) then return false
		var fe = self.elements[f]
		return fe.tos.has(t)
	end