diff --git a/Missing.md b/Missing.md
index b43c0db5f7bbd604f1eb0c31543f377c5f56770c..76971ab8accc7aa8713ce91a7692959fffecc03d 100644
--- a/Missing.md
+++ b/Missing.md
@@ -5,6 +5,8 @@ Missing APIs from the types we cover (APIs have been added after this formalizat
 * Structural conversion for slices.  The matching operations in our model would be
   `&mut Cell<(A, B)>` -> `&mut (Cell<A>, Cell<B>)` and
   `&Cell<(A, B)>` -> `&(Cell<A>, Cell<B>)` (both being NOPs).
+  * Turns out to be very hard!  The way we currently associate NA-masks with locations is in conflict with this.
+    The invariant for the entire cell gets allocated "on" the first location of the cell, so when we do splitting the 2nd projection has no way to access it...
 
 # ZST