diff --git a/README.md b/README.md
index cb061fb..80a5674 100644
--- a/README.md
+++ b/README.md
@@ -14,6 +14,7 @@ This is nowhere near complete. Stay tuned.
 
  - Replace strings with &str?
  - Function application
+ - Errors on bad values (mod negative, rt -1)
  - Commands
  - Documentation (usage)
  - Documentation (comments)