なんでこうなるのかわからない

undef_and_extend.rb

module ModA
  def methodA
    puts "methodA of ModA called."
  end
end

module ModB
  include ModA
  undef methodA
end

class ClassA
  
  def methodA
    puts "methodA of ClassA called."
  end

end

a = ClassA.new
a.methodA      # => methodA of ClassA called.
a.extend ModB
a.methodA      # => NoMethodError

undef_and_include.rb

module ModA
  def methodA
    puts "methodA of ModA called."
  end
end

module ModB
  include ModA
  undef methodA

end

class ClassA

  include ModB
  
  def methodA
    puts "methodA of ClassA called."
  end

end

class ClassB

  def methodA
    puts "methodA of ClassB called."
  end

end

a = ClassA.new
a.methodA      # => methodA of ClassA called.

b = ClassB.new
class << b
  include ModB
end
b.methodA      # => NoMethodError