class_eval &&

class Foo1
  
end

def Foo1.foo
  puts "foo"
end


class Foo2
  def self.foo
    puts "foo"
  end
end


class Foo3
  
end

class << Foo3
  def foo
    puts "foo"
  end
end

class Foo4
  class << self
    def foo
      puts "foo"
    end
  end
end


class Foo5
  
end

Foo5.class_eval do 
  def self.foo
    puts "foo"
  end
  
end


class Foo6
  
end

Foo6.instance_eval do
  def foo
    puts "foo"
  end
end