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.

Doc/texinputs/myindex.ist

@@ -3,7 +3,7 @@ headings_flag 1
 heading_prefix "  \\bigletter "
 
 preamble "\\begin{theindex}
-\\def\\bigletter#1{{\\Large\\sffamily#1}\\vspace{1mm}}
+\\def\\bigletter#1{{\\Large\\sffamily#1}\\nopagebreak\\vspace{1mm}}
 
 "