diff --git a/cyclone.h b/cyclone.h
index 2d61eb68..04d94a66 100644
--- a/cyclone.h
+++ b/cyclone.h
@@ -87,6 +87,10 @@ typedef long tag_type;
 #define boolean_tag 15
 #define cvar_tag 16
 #define vector_tag 17
+/* TODO: need a type to return from things like
+   (if #f #t) and (write 1) so the REPL will not print
+   anything for them:
+  void_tag */
 
 #define nil NULL
 #define eq(x,y) (x == y)