gistfile1.rb

module A
	def A.included(klass)
		klass.instance_exec do
			@@foo = 0
			
			def inc()
				puts self.object_id
				
				puts @@foo
				
				@@foo += 1
				
				puts @@foo
			end
			
			def foo()
				puts self.object_id
				puts @@foo
			end
		end
	end
end

class B
	include A
	
	inc()
	inc()
end

class C
	include A
	
	inc()
end

B.foo()
C.foo()

# OUTPUT #
69875669805760
0
1
69875669805760
1
2
69875669805620
0
1
69875669805760
1
69875669805620
1

The desired output is:
69875669805760
0
1
69875669805760
1
2
69875669805620
0
1
69875669805760
2
69875669805620
1