diff --git a/day09/sol.tcl b/day09/sol.tcl
index 05c8641..b0af115 100755
--- a/day09/sol.tcl
+++ b/day09/sol.tcl
@@ -26,6 +26,8 @@ proc predict {lst} {
 }
 
 proc unpredict {lst} {
+    # note: this could be
+    #   return [predict [lreverse $lst]]
     if {[allzeros $lst]} {
         return 0
     }