> > They contain code that looks like Lisp but does not have the same
  > > semantic rules as Lisp.

  > Yes: it's a domain-specific language.

THat is why pcase adds to the total complexity of Emacs Lisp: because
it incorporates a domain-specific language.  (I called it a "little
lajguage".)

  > To interpret those backquotes and commas, one can imagine that the they
  > do the inverse of the usual.

I uderstand that when you say it, but when I saw the line

>          > (`(,hookfun . (,start ,end ,collection . ,plist))

I was not sure which part of the line that rule applies to.

In particular, what is the reason for the period and inner
parenthsses?  Why doesn't the code say this?

>          > (`(,hookfun ,start ,end ,collection . ,plist)

Would that be equiva;ent?

I could not assume the inner parenthsses were unnecessary.  Maybe
there was a conventiob by which they change the meaning.
I did not know.