diff --git a/TODO.md b/TODO.md
index 3596bbe..e71e136 100644
--- a/TODO.md
+++ b/TODO.md
@@ -8,6 +8,7 @@
    - Better error when `sin = 2`
    - Check syntax when parsing or when evaluating?
  - list and delete variables
+ - redefine variables
 
 ## Pre-release
  - Commands to list constants, units, and substitutions