I don't use vim, but from the parsing example he gives in the post, I'm guessing he uses C-c:

    `Mihere's some text^Cyyp$bimore ^C0~A.^C:w^M:q

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> The lexer correctly determined that I started in normal mode by navigating to a specific buffer using the `M mark, then typed here's some text in insert mode, then[…]