explain replay prevention in obfs4 spec
phw informs me that obfs4 does indeed have replay prevention (where if you record the client traffic and send it again, you don't get to learn if it's an obfs4 bridge). But when I look at https://gitweb.torproject.org/pluggable-transports/obfs4.git/tree/doc/obfs4-spec.txt then the word 'replay' doesn't show up.
How does the protection work? Should it be in the spec?
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