Shows that a module instance method will not overwrite a class instance method of the same name if `include` is used, but *will* if `prepend` is used.

module-method-doesnt-override-class-instance-method.rb

#!/usr/bin/env ruby

module M
  def foo
    puts 'I am a module instance method.'
  end
end

# Class whose foo instance method is defined *after* the include.
class C1
  include M

  def foo
    puts 'I am a class instance method.'
  end
end

# Class whose foo instance method is defined *before* the include.
class C2
  def foo
    puts 'I am a class instance method.'
  end

  include M
end

# Class whose foo instance method is defined *after* the include.
class C3
  prepend M

  def foo
    puts 'I am a class instance method.'
  end
end

# Class whose foo instance method is defined *before* the include.
class C4
  def foo
    puts 'I am a class instance method.'
  end

  prepend M
end

C1.new.foo #=> I am a class instance method.
C2.new.foo #=> I am a class instance method.
C3.new.foo #=> I am a module instance method.
C4.new.foo #=> I am a module instance method.