Index formatting customization is achieved through registering a conversion
function for @printindex (see Command Tree Element Conversion Functions). The Texinfo parsed document index entries information directly
obtained from the Texinfo manual parsing is available through
$converter->get_info('document')->indices_information()
(see Converter General Information). Sorted index entries, which are
usually used for index formatting are available through
get_converter_indices_sorted_by_letter:
\%sorted_index_entries = $converter->get_converter_indices_sorted_by_letter () ¶Returns index entries sorted by index and letter.
This function should be called each time sorted indices are needed,
in case the sorting depends on the @documentlanguage value.
See Texinfo::Convert::Converter::get_converter_indices_sorted_by_letter.