After taking a look at the definition of
byte-run.el, it seems that the macro does some very interesting things regarding checking for the existence of a docstring.
First, it uses a
declare form to indicate that the third argument to the
defun is the docstring. It then checks whether the third argument is a string (which means it's a docstring), or actually a
declare form (which means there is no docstring).
If I understand correctly, this allows the user of a
defun to specify a docstring or to it out altogether.
Is this use of an optional argument considered unusual? Usually, for a function
(foo a b &optional c d &rest rest), we'd enter a
c if we wanted to specify
d but not
c. I haven't seen other examples of arguments that can simply be "left out" like the docstring.