comma-comma-at in Common Lisp

I am confused about how comma-comma-at does what it does.

An example of use of comma-comma-at is in Is there a use for double unquote (double comma) when defining a Lisp macro?

It seems to me that

(let ((parms '(x y)))
  ``(beg ,,@parms end))

expands to or something equivalent to

`(beg ,x ,y end)

How does it expands to that?

I was thinking, if I evaluate the double backquote form, it causes the second comma to do its work, and the result is:

`(beg ,<splice stuff here> end)

and it looks like Lisp interpreter should complain and say "I don't know what it means to splice stuff before a comma" in the sense that when Lisp encounters

`,@abc

it would be likely to say "I don't know what it means to splice stuff before a backquote. Don't ever do that again."

But somehow, the interpreter does not complain and simply choose to do

`(beg <splice stuff here and write comma in front of each of them> end)

Is that compatible with rules in CLHS backquote?

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Also see CHLS "innermost backquoted form should be expanded first" meaning

CLHS states: "If the backquote syntax is nested, the innermost backquoted form should be expanded first. This means that if several commas occur in a row, the leftmost one belongs to the innermost backquote."

So, the first comma unquotes the second backquote; the ,@ then splices in the contents of parms, as all quotes have been undone. Unquoting does not "replace" forms in steps as you describe - the commas just back up in the structure of backquoted forms when deciding how to evaluate the contained forms.

(let ((parms '(x y)))
  `(list 'beg ,@parms 'end))

obviously evaluates to (list 'beg x y 'end), which is what you are after here; producing a form that creates the desired structure as its output. The additional outer backquote is simply a shorthand for this syntax, in the same manner as '(foo bar) is (equality issues aside) a shorthand for (list 'foo 'bar).