New index entries.
diff --git a/Doc/lib/libsys.tex b/Doc/lib/libsys.tex
index 932520d..3138180 100644
--- a/Doc/lib/libsys.tex
+++ b/Doc/lib/libsys.tex
@@ -176,6 +176,8 @@
 
 \begin{datadesc}{ps1}
 \dataline{ps2}
+\index{interpreter prompts}
+\index{prompts, interpreter}
   Strings specifying the primary and secondary prompt of the
   interpreter.  These are only defined if the interpreter is in
   interactive mode.  Their initial values in this case are