How to delete all odd lines
ref: https://jingyan.baidu.com/article/fea4511ad3b199f7bb9125ba.html
:%normal jkdd # even lines from current line (current line is 0)
:%normal jdd # odd lines
ref: https://jingyan.baidu.com/article/fea4511ad3b199f7bb9125ba.html
:%normal jkdd # even lines from current line (current line is 0)
:%normal jdd # odd lines