Language Design Is Not Just Solving Puzzles (2006)
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Besides, this is a language that likes to surround special identifiers with double underscores, which is almost objectively ugly.
Another thing, reading closely I think what he wanted to say (but maybe failed to communicate clearly) is that the double colon is a problematic lexical choice because in the slice syntax, it's already possible to have two colons in a row, but these should be lexed as separate tokens!