t / t4101 / diff.3-1on commit sequencer.c: teach append_signoff how to detect duplicate s-o-b (bab4d10)
   1--- a/frotz
   2+++ b/frotz
   3@@ -1,3 +1,3 @@
   4 a
   5+b
   6 c
   7-b
   8\ No newline at end of file