Next on the tour of uncommon Lisp features is function-keywords.  I can’t really think of a time when a programmer might need to use this, it is a function that one typically would use in the implementation of the generic function framework in Common Lisp, as part of the system to determine whether a particular method matches a particular generic function definition.

If any reader can find a context, outside of implementing Lisp, when the programmer might need to use this, please let me know in the comments.