Yes, I just know subseq. And there are also seq-subseq, seq-take, seq-drop, butlast, Two many and hard to remmber.
Almost all of them are implemented in lisp except copy-sequence which is in C. Some of them are implemented
by copying the whole sequence and then modify the copy. If we add the two args to copy-sequence, 
then  these functions can be based on it with only one or two lines in lisp, and reduce some data copy.
Does the two args make copy-sequence too much complex internally?




在 2017-12-15 16:24:50，"Elias Mårtenson" <lokedhs@gmail.com> 写道：

On 15 December 2017 at 15:54, John Wiegley <johnw@gnu.org> wrote:
>>>>> "j" == jun  <netjune@163.com> writes:

j> like this: (copy-sequence seq &optional from to). 'from' and 'to' mean the
j> same as in the function 'substring'.

I'd rather not overload copy-sequence that way, and rather have a new function
named (sublist LIST &optional FROM TO), even if copy-sequence xs == sublist xs.



There is already the function ‘subseq’ that does exactly this, or did I misunderstand
the question?