diff --git a/TODO b/TODO
index d279edb..e0e5aab 100644
--- a/TODO
+++ b/TODO
@@ -1,3 +1,7 @@
+non-trivial space optimization
+(For each state whose word_space is free, set a proper value such that it may cover most whitespaces)
+(Or just set word_space according to the first positive whitespace, but need to do this before the state inherit some value)
+
 == Future: ==
 
 Too difficult/complicated to implement: