Make <|> for ZipLists lazy for infinite lists

[박신환]

Current definion of (<|>) for ZipLists:
ZipList xs <|> ZipList ys = ZipList (xs ++ drop (length xs) ys)

doesn't work if the left argument is infinite. It should be:

ZipList [] <|> ys = ys
xs <|> ZipList [] = xs
ZipList (x:xs) <|> ZipList (_:ys) = ZipList (x : (ZipList xs <|> ZipList ys))
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