RE: [External] : Handling nil values in optional argument

[Drew Adams]

> I have the following tool, but I want that if I pass nil in the third
> argument,
> the function does not assume that fyr was not supplied, but that fyr is
> actually
> to be used as a nil value.
> 
> (defun mundu (monbf funcs labels &optional fyr)
>   (unless fyr (setq fyr t))

You can't.  nil is the default value for an optional argument.

Instead, your code can use something other than nil for the value that you pass 
as an explicit argument.  It can use the singleton list (nil), for example.