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 The reason is same to why we use interleave in 3.5.3, it's convenient to display infinite stream. 


 Interleaving is like merging in traffic. You take one from one lane and one from the other in order to form a zipper. Otherwise you risk a person sitting at the front of one lane never turning and being honked at for all of time. That is one of your infinite lists if you attempt to use append instead of interleave. For some reason, Californians never figured out merging in traffic either...