Challenge 2: Shared Behavior, No Common Ancestor — Solution

module Loggable
  def log(message)
    puts "[LOG] #{message}"
  end
end

class Order
  include Loggable
end

class Invoice
  include Loggable
end

Order.new.log("Order created")
Invoice.new.log("Invoice generated")

=begin
Output:
[LOG] Order created
[LOG] Invoice generated

Notes:
- Order and Invoice share no inheritance relationship at all — neither is
  a subclass of the other, and both ultimately just inherit from Object
  like every Ruby class does.
- Both classes independently include the same Loggable module, and both
  get an identically-behaving log method as a result.
- This is the exact mechanism Chapter 5's Array and Hash rely on to share
  map/select/reduce via the Enumerable module, despite Array and Hash also
  having no inheritance relationship to each other.
=end