+The option is expanded before use.
+.new
+During the expansion &$auth1$& is set with the client username,
+&$auth2$& with the iteration count, and
+&$auth3$& with the salt.
+
+The intent of this option
+is to support clients that can cache thes salted password
+to save on recalculation costs.
+The cache lookup should return an unusable value
+(eg. an empty string)
+if the salt or iteration count has changed
+
+If the authentication succeeds then the above variables are set,
+.vindex "&$auth4$&"
+plus the calculated salted password value value in &$auth4$&,
+during the expansion of the &%client_set_id%& option.
+A side-effect of this expansion can be used to prime the cache.
+.wen