[racket] Peekable asynchronous channel?

I'd like to have an asynchronous channel that does not remove a value from
its buffer on sync, so that I can separate the code that waits for a
message from the code that removes everything currently in the channel. In
some sense, I want sync to do a "peek", not a "get". Does anything like
this exist in Racket (or perhaps in the package system), or would I have to
write it myself?

Jonathan
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