You are right. I can implement my own 
method to return objects that are not vector, string or list.

Still, it would be convenient to have a function
that can destructively modify a sequence. What is
the reason to not have one?

Yuan

On Sun, Dec 23, 2018 at 9:34 AM Stefan Monnier <monnier@iro.umontreal.ca> wrote:
>>   (seq-concatenate 'list (seq-subseq seq 0 n)
>>                          (list element)
>>                          (seq-subseq seq n))
>
> That returns a list, not the sequence.

I'm not sure which sequence is "the" sequence, but a list is
definitely a sequence.


        Stefan