Added doco for abstract interface
diff --git a/Doc/ext.tex b/Doc/ext.tex
index 963f8ff..6603eb0 100644
--- a/Doc/ext.tex
+++ b/Doc/ext.tex
@@ -1364,6 +1364,8 @@
 files).  No \samp{-l} options can be used.
 
 
+\input{extref}
+
 \input{ext.ind}
 
 \end{document}