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Add a field-handling capability, so sorting may be done on fields within lines, each field sorted according to an independent set of options. (The index for this book was sorted with -df for the index category and -n for the page numbers.)

/* This seems like altogether too much work. I can see how to do it, and started an
 * implementation; but I place higher value on progressing further through the book than
 * completing this challenge. I am confident I could do it, though it would take me a
 * little while. */