-If this option may be omitted then the hash is read from the cache file;
-if there is no cache file either, then an error is reported.
+This option may be omitted: if it is, then the hash is read from the
+cache file; if there is no cache file either, then an error is reported.
+.TP
+.B \-u, \-\-udiff
+Rather than produce a manifest, read a unified
+.BR diff (1)
+from standard input, and clear from the cache all files mentioned as
+being different. Filenames in the diff are considered relative to
+.I dir ,
+defaulting to the current working directory.