Think of a physical zipper, where the two sides with the teeth come together to interlock... xs = [1,2,3,4,5,6,... ys = [a,b,c,d,e,f,... [ (1,a),

Think of a physical zipper, where the two sides with the teeth come together to interlock... xs = [1,2,3,4,5,6,... ys = [a,b,c,d,e,f,... [ (1,a), (2,b), (3,c), (4,d), (5,e), |6| |f| |7| |g| |8| |h| |9| |i| |0| |j| . . . . . . ] https://kezan.eu/wp-content/uploads/2016/05/Zipper_Thumb_1440.jpg https://kezan.eu/wp-content/uploads/2016/05/Zipper_Thumb_144...