Is there a best way to make this work?

default_args_example.rb

module DefaultArgs
  NOT_SET = Module.new
  private def set?(value) = value != NOT_SET
  private def not_set?(value) = !set?(value)
    
  # This is the solution that I came up with:
  # def self.extended(base)
  #  base.const_set :NOT_SET, NOT_SET
  #  super
  # end
end

class OnInstance
  include DefaultArgs
  def call(a: NOT_SET) = set?(a)
end

class OnClass
  extend DefaultArgs
  def self.call(a: NOT_SET) = set?(a)
end

OnInstance.new.call      # => false
OnInstance.new.call a: 1 # => true
OnClass.call             # => false
OnClass.call a: 1        # => true