+/*
+ * TODO: On return, we can use a more efficient
+ * untagging (we know the constructor tag).
+ *
+ * When entering stg_sel_#_upd, we know R1 points to its closure,
+ * so it's untagged.
+ * The payload might be a thunk or a constructor,
+ * so we enter it.
+ *
+ * When returning, we know for sure it is a constructor,
+ * so we untag it before accessing the field.
+ *
+ */