Tobias C Rittweiler wrote:

Actually the point is to _improve_ readability by specifying
intent: the macro writer would indicate to both, the human 
and Lisp reader, that the following part of the expansion
is optional.

"Unfamiliarity", and "unportability" are in the very nature
of extensions. What's worth arguing about is whether or not,
or rather in how far, an extension is "in the spirit of CL";
Well, it's in the spirit of CL in the same way that
all those complicated "format" directives are,
namely, CL's design didn't particularly try to
keep things simple.

But that aside, yes, it's consistent with the
idea of backquote.  It makes sense as
something to add that's in line with the
original intent (rather than being some
kind of add-on that has no business
being there).  One could reasonably
argue that if we were to add one more
thing to backquote, this would be the
best one to add.

My experience has been that backquote is
somewhat hard to grasp, and that adding
more stuff to it is dicey.  My own feeling
is that it would be better not to add this.

Don't get me wrong: I certainly think that
reasonable people can disagree about this.
It's just a matter of taste and a fine distinction.
There's no "right" answer.

And also, we're not discussing a lanaguage
standard, as you point out.  And CL has
benefited from experimentation, and it's
hard to do a meaningful experiment with
langauge design unless you implement it,
document it, and see what programmers do.
That's how so much stuff got added to CL
and its predecessors.

-- Dan
in how far a feature is likely to be taken up by an 
experienced Common Lisp programmer easily. I'd argue that
this is the case for ,?, but you may disagree with it.

Quick quiz: What does `(,.x .,x) do?

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