seqnum.rb

#!/usr/bin/env ruby

class A
  class << self
    def next_sequence_number
      @next_sequence_number += 1
    end
  end

  @next_sequence_number = 0

  attr_reader :sequence_number

  def initialize
    @sequence_number = self.class.next_sequence_number
  end
end

10.times { p A.new.sequence_number }