Sometimes you will want to combine two disparate indices such as functions and concepts, perhaps because you have few enough of one of them that a separate index for them would look silly.
You could put functions into the concept index by writing
@cindex commands for them instead of @findex commands,
and produce a consistent manual by printing the concept index with the
title `Function and Concept Index' and not printing the `Function
Index' at all; but this is not a robust procedure. It works only if
your document is never included as part of another
document that is designed to have a separate function index; if your
document were to be included with such a document, the functions from
your document and those from the other would not end up together.
Also, to make your function names appear in the right font in the
concept index, you would need to enclose every one of them between
the braces of @code.