diff --git a/book/external/TeachBooks b/book/external/TeachBooks
index f6968d6..21a67f7 160000
--- a/book/external/TeachBooks
+++ b/book/external/TeachBooks
@@ -1 +1 @@
-Subproject commit f6968d6284d5f75950e502c50c6778d08d0fdf1f
+Subproject commit 21a67f72539aa144de54bf05b9b8d24771590771