Combining **&optional** and **&key** parameters yields surprising enough results that you should probably avoid it altogether. The problem is that if a caller doesn’t supply values for all the optional parameters, then those parameters will eat up the keywords and values intended for the keyword parameters. For instance, this function unwisely mixes **&optional** and **&key** parameters:

If called like this, it works fine:

But this will signal an error:

This is because the keyword is taken as a value to fill the optional y parameter, leaving only the argument 3 to be processed. At that point, Lisp will be expecting either a keyword/value pair or nothing and will complain. Perhaps even worse, if the function had had two **&optional** parameters, this last call would have resulted in the values :z and 3 being bound to the two **&optional** parameters and the **&key** parameter z getting the default value **NIL** with no indication that anything was amiss.

You can safely combine and **&key** parameters, but the behavior may be a bit surprising initially. Normally the presence of either **&rest** or **&key** in a parameter list causes all the values remaining after the required and **&optional** parameters have been filled in to be processed in a particular way—either gathered into a list for a **&rest** parameter or assigned to the appropriate **&key** parameters based on the keywords. If both **&rest** and **&key** appear in a parameter list, then both things happen—all the remaining values, which include the keywords themselves, are gathered into a list that’s bound to the **&rest** parameter, and the appropriate values are also bound to the **&key** parameters. So, given this function:

you get this result: