RE: (copy-marker nil)

[Drew Adams]

> > For most functions, passing nil for an &optional parameter is the same
> > as omitting it.
> > If MARKER is not specified, the new marker does not point anywhere.
> 
> It's about the return value.
> A marker pointing nowhere, might it be considered valid?

What do you mean by "valid"?  Yes, it's a marker.  No, it
doesn't have a buffer or position.  The same is true of
other Emacs-Lisp objects, such as overlays: they can exist
without having non-nil values for certain of their properties.

> Expected such returning nil.

Why?  The doc string is pretty clear that it returns a
marker.  And it is clear what happens if arg MARKER is
nil: "the new marker does not point anywhere".