+ "Initiate a sign operation on STRING.
+
+If optional 3rd argument MODE is 'clearsign, it makes a clear text signature.
+If MODE is t or 'detached, it makes a detached signature.
+Otherwise, it makes a normal signature.
+
+If you use this function, you will need to wait for the completion of
+`epg-gpg-program' by using `epg-wait-for-completion' and call
+`epg-reset' to clear a temporaly output file.
+If you are unsure, use synchronous version of this function
+`epg-sign-string' instead."