s/a/b/ v "voids the substitution zv "undoes the V, so substitution is back v "voids the ZV, so sub is undone again
ZV undoes the effects of the most recent V command. ZV will issue an error if there have been any active commands issued since the last V. ("expl fred v" explains the difference between active and passive commands.) V can be used to undo the effects of ZV.
ZV does not affect the current pattern //. Thus the current pattern will always be the one most recently used in a FRED command, even if the effects of that command have been undone with V or ZV.
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