+
+ # Two posets are equal if they contain the same elements and edges.
+ #
+ # ~~~
+ # var pos1 = new POSet[String]
+ # pos1.add_chain(["A", "B", "C", "D", "E"])
+ # pos1.add_chain(["A", "X", "C", "Y", "E"])
+ #
+ # var pos2 = new POSet[Object]
+ # pos2.add_edge("Y", "E")
+ # pos2.add_chain(["A", "X", "C", "D", "E"])
+ # pos2.add_chain(["A", "B", "C", "Y"])
+ #
+ # assert pos1 == pos2
+ #
+ # pos1.add_edge("D", "Y")
+ # assert pos1 != pos2
+ #
+ # pos2.add_edge("D", "Y")
+ # assert pos1 == pos2
+ #
+ # pos1.add_node("Z")
+ # assert pos1 != pos2
+ # ~~~
+ redef fun ==(other) do
+ if not other isa POSet[nullable Object] then return false
+ if not self.elements.keys.has_exactly(other.elements.keys) then return false
+ for e, ee in elements do
+ if ee.direct_greaters != other[e].direct_greaters then return false
+ end
+ assert hash == other.hash
+ return true
+ end
+
+ redef fun hash
+ do
+ var res = 0
+ for e, ee in elements do
+ if e == null then continue
+ res += e.hash
+ res += ee.direct_greaters.length
+ end
+ return res
+ end