src/Pure/Thy/bibtex.scala

 
       Isabelle_System.bash(
         "\"$BIB2XHTML_HOME/main/bib2xhtml.pl\" -B \"$ISABELLE_BIBTEX\"" +
           " -u -s " + Bash.string(proper_string(style) getOrElse "empty") +
           (if (chronological) " -c" else "") +
-          (if (title != "") " -h " + Bash.string(title) + " " else "") +
+          if_proper(title, " -h " + Bash.string(title) + " ") +
           " " + File.bash_path(in_file) + " " + File.bash_path(out_file),
         cwd = tmp_dir.file).check
 
       val html = File.read(tmp_dir + out_file)