can't understand what's happening when adding a property to a symbol

That's because you are using '(nil) to create the initial disembodied property list, and you're doing so in a loop. That means that the AAA, BBB, CCC properties on the trial symbol are all pointing to exactly the same list in memory - it's actually the literal list in the code. You are then destructively modifying that list with the final putpropq, and all three are still pointing at the same list. 

You can see this if you do:

eq(trial->AAA trial->BBB) => t

Similarly, if you do this:

(defun setThreeProps ()
  (foreach key '(AAA BBB CCC)
    (putprop trial '(nil) key)
  ))

(setq trial (makeSymbol "toto"))
(setThreeProps)
(putpropq (getq trial AAA) t test)

now do (pp setThreeProps) to see the code:

procedure(setThreeProps()
    foreach(key
        '(AAA BBB CCC)
        putprop(trial
            '(nil test t) key
        )
    )
)

(of course, I could have pretty-printed it in non-infix mode if I'd done (sstatus printinfix nil) first). Anyway, the key is you'll see that the literal list in the code has also been updated - that's the literal list that the quote operator generates and everything points to.

Very simply you can fix this by using (ncons nil) or (list nil) in the loop instead:

(foreach key '(AAA BBB CCC)
  (putprop trial (ncons nil) key)
)

This way it is guaranteed that the list for each key is newly and separately created, and is not shared.

Hope that makes sense!

Andrew

Thank you, it's so much clear now.

I've also found an answer to a case about the same kind of issue.

I share it for community people interested in that kind of topic.

support.cadence.com/.../ArticleAttachmentPortal

kkdesbois said:

I share it for community people interested in that kind of topic.

support.cadence.com/.../ArticleAttachmentPortal

Ah, that's one of my articles! I thought I remembered writing this up for Cadence Online Support!

Andrew