diff --git a/chapter09.tex b/chapter09.tex
index 9121590..63eb4c0 100644
--- a/chapter09.tex
+++ b/chapter09.tex
@@ -744,9 +744,10 @@ takes $O(1)$ time using bit operations.
 
 \index{segment tree}
 
-A \key{segment tree}\footnote{The origin of this structure is unknown.
+A \key{segment tree}\footnote{Quite similar structures were used
+in the late 1970's to solve geometric problems \cite{ben80}.
 The bottom-up-implementation in this chapter corresponds to
-the implementation in \cite{sta06}.} is a data structure
+that in \cite{sta06}.} is a data structure
 that supports two operations:
 processing a range query and
 modifying an element in the array.