# Write the POSet as a graphviz digraph.
	#
	# Nodes are labeled with their `to_s` so homonymous nodes may appear.
	# Edges are unlabeled.
	fun write_dot(f: Writer)
	do
		f.write "digraph \{\n"
		var ids = new HashMap[E, Int]
		for x in elements.keys do
			ids[x] = ids.length
		end
		for x in elements.keys do
			var xstr = (x or else "null").to_s.escape_to_dot
			var nx = "n{ids[x]}"
			f.write "{nx}[label=\"{xstr}\"];\n"
			var xe = self.elements[x]
			for y in xe.dtos do
				var ny = "n{ids[y]}"
				if self.has_edge(y,x) then
					f.write "{nx} -> {ny}[dir=both];\n"
				else
					f.write "{nx} -> {ny};\n"
				end
			end
		end
		f.write "\}\n"
	end