diff --git a/umbilical/frozencas/frozencas.go b/umbilical/frozencas/frozencas.go
index dfe8894..1bf4de8 100644
--- a/umbilical/frozencas/frozencas.go
+++ b/umbilical/frozencas/frozencas.go
@@ -11,6 +11,9 @@
 //	| index  |
 //	+--------+
 //
+// We put the index after the data so that we write the data first, and
+// compute the index afterwards.
+//
 // The header is given by.
 //
 //	+---------------------+--------------+