diff --git a/book/preamble.tex b/book/preamble.tex
index da6c30c..3ccc860 100644
--- a/book/preamble.tex
+++ b/book/preamble.tex
@@ -45,6 +45,8 @@
     linkcolor=blue		%choose some color if you want links to stand out
 }
 
+\usepackage{scrextend}		% so can use \footref
+
 % don't say subsection and subsubsection in auto-references
 \let\subsectionautorefname\sectionautorefname
 \let\subsubsectionautorefname\sectionautorefname