diff --git a/fmmdset/fmmdset.tex b/fmmdset/fmmdset.tex
index 002963a..e09d727 100644
--- a/fmmdset/fmmdset.tex
+++ b/fmmdset/fmmdset.tex
@@ -268,6 +268,8 @@ real time control systems often have a small number of major reportable faults (
 even though they may have accompanying diagnostic data.
 
 
+The hierarchy is of the form of a directed acyclic graph. It is permissible, for instance, to
+create a functional group from components at different levels of ifailure mode abstraction.
 
 \cite{sem}