Switch to Emacs.
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​Just kidding!
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A Leanpub author shared this:

I just want to share a little fix I made to the markdown syntax in order to make it working with the code markup that this wonderful service requires when you want to insert a snippet of code in your books.
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Just create a markdown.vim file in your .vim/after/syntax dir and add the following lines:

syn match markdownLeanCode '^<<(.*)$'
hi def link markdownLeanCode markdownH1
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Now your code snippets won't mess up with the official markdown syntax provided by Vim