wtf.rb

#!/usr/bin/env ruby

module A
  def self.included(base)
    base.include InMethods
  end

  module InMethods
    def mem
      @mem ||= []
    end

    def add(n)
      mem += n
    end
  end
end

class Help
  include A
end

h = Help.new
h.add(1)
puts(mem: h.mem)

__END__

error-included-modules-accessing-eachother-errors.rb:14:in `add': undefined method `+' for nil:NilClass (NoMethodError)
	from error-included-modules-accessing-eachother-errors.rb:24:in `<main>'