Fix suggested by Lorenzo M. Catucci <lorenzo@argon.roma2.infn.it> to keep the
large letter at the top of each index section with at least the first entry
for that letter.
diff --git a/Doc/texinputs/myindex.ist b/Doc/texinputs/myindex.ist
index bfc2e34..9ffa0f9 100644
--- a/Doc/texinputs/myindex.ist
+++ b/Doc/texinputs/myindex.ist
@@ -3,7 +3,7 @@
heading_prefix " \\bigletter "
preamble "\\begin{theindex}
-\\def\\bigletter#1{{\\Large\\sffamily#1}\\vspace{1mm}}
+\\def\\bigletter#1{{\\Large\\sffamily#1}\\nopagebreak\\vspace{1mm}}
"