汎用の最大値・最小値

ruby-list にも流してみたもの
coerce がミソ

class Sentinel
	include Comparable

	MAX = new

	def MAX.<=> other
		1
	end

	def MAX.coerce other
		[MIN, self]
	end

	MIN = new

	def MIN.<=> other
		-1
	end

	def MIN.coerce other
		[MAX, self]
	end

	private

	def self.new
	end
end