Included Ruby modules go into the inheritance chain

mixin_ancestors.rb

irb(main):001:0> module Mixin; end
=> nil
irb(main):002:0> class A
irb(main):003:1> include Mixin
irb(main):004:1> end
=> A
irb(main):005:0> a = A.new
=> #<A:0x007fbaea8a89a8>
irb(main):006:0> a.is_a? Mixin
=> true
irb(main):007:0> a.class.ancestors
=> [A, Mixin, Object, Kernel, BasicObject]
irb(main):008:0> a.is_a? Mixin
=> true