How do I know if an unbuffered (0 sized) channel ready to put! (not blocking)?

General Usage
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1

For a buffered channel(n) with n>0, I can check it full or not with comparing the Channel field: sz_max with the undocumented function Base.n_avail(channel).

For channel(0), when a take! on it, a put! can be operated on this 0-sized channel without blocking. How do I know if channel(0) is NOT blocking to put! ?

Much THX