diff --git a/Bookshelf/Enderton/Set.tex b/Bookshelf/Enderton/Set.tex
index 89e8447..137c4ca 100644
--- a/Bookshelf/Enderton/Set.tex
+++ b/Bookshelf/Enderton/Set.tex
@@ -9604,7 +9604,7 @@
     \paragraph{($\Leftarrow$)}%
 
       Suppose $\ran{f} = A$.
-      TODO
+      TODO: by induction?
 
   \end{proof}