gistfile1.rb

# On include we're grepping method baz from instance methods 
module Foo
  def self.included(base)
    puts base.instance_methods.grep(/baz/).inspect
  end
end

class Bar
  include Foo
  def baz; end
end
# prints [] -- no method baz found!

class Bar
  include Foo
end
# prints ["baz"] -- now that we re-opened same class method already there

class Qux
  def baz; end
  include Foo
end
# prints ["baz"] -- since baz is declared before inclusion of Foo