behaviour change in cl-subseq

[Phillip Lord]

I've just had a bug report for pabbrev. The root cause is a change in
the implementation of cl-subseq. 

In Emacs 24:

(cl-subseq '() 10 10) ;; nil

While in Emacs 25

In Emacs 25 `cl-subseq' has been redefined in terms of seq-subseq.

(seq-subseq '() 10 10) ;; errors
(cl-subseq '() 10 10) ;; errors

Which is a reasonably substantial change in the interface of cl-subseq.
The actual (exceptional) behaviour of cl-subseq is not documented, so
it's reasonable, but perhaps not sensible. It's easy to fix in pabbrev
(and anywhere) in a way which does not require me to probe for Emacs
versions.

I offer no opinions, just wanted to check whether this was intended.

Phil