Ruby：C类包含模块M;包括M中的模块N不影响C.什么给出？

Array是一个默认包含Enumerable的类.然而,它并没有受到Narf在模块中的后期加入的影响.

有趣的是,在包含之后定义的类从Enumerable获得Narf.

例：

# This module provides essential features
module Narf
  def narf?
    puts "(from #{self.class}) ZORT!"
  end
end

# I want all Enumerables to be able to Narf
module Enumerable
  include Narf
end

# Fjord is an Enumerable defined *after* including Narf in Enumerable
class Fjord
  include Enumerable
end

p Enumerable.ancestors    # Notice that Narf *is* there
p Fjord.ancestors         # Notice that Narf *is* here too
p Array.ancestors         # But,grr,not here
# => [Enumerable,Narf]
# => [Fjord,Enumerable,Narf,Object,Kernel]
# => [Array,Kernel]

Fjord.new.narf?   # And this will print fine
Array.new.narf?   # And this one will raise
# => (from Fjord) ZORT!
# => NoMethodError: undefined method `narf?' for []:Array