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Right now double colon is allowed only when used as a start of not empty identifier prefix. I wonder is there some explanation why it is forbidden anywhere else?
I found original commit, where this was introduced. https://github.com/clojure/clojure/commit/005ea1b5f96c5bb762e155032a865e29ad71bcf3
But commit message has no explanation why it allows only non-repeating colon
