This file is in the public domain. The -a flag tells idxtext to abort if an index file already exists. The first line of the index file is a pointer to the last indexed byte of the text-base file it indexes.
Index files are too large.
Also, I've yet to find a portable way of creating unique index names that are capable of being uniquely identified with their original text file. It might be sensible to hard code the name into the index. The chances of a conflict seem remote enough that I haven't bothered. If you're worried, use the -a flag.