(seq-subseq) exists in 24.5, but not in 26.3?

[Douglas Lewan <d.lewan2000@gmail.com>]

I've started looking at LEAN, and, of course, I want lean-mode. I've 
managed to overcome many hurdles, typically dependencies of one sort or 
another. (If you're running Linux, you've been there too.)

However, somehow it's having a hard time finding (seq-subseq), which 
certainly looks like it should be defined. FYI (seq-subseq) is wanted by 
flycheck, although I suppose there could be other things that want it. 
The error that gets signaled is this: Debugger entered--Lisp error: 
(void-function seq-subseq).

I'm running Debian, emacs version 26.3, it has feature seq, and 
correspondingly feature seq-25, which looks like it defines (seq-subseq).

Babbling background that you can ignore if you want follows

When I run emacs 24.5, (seq-subseq) is defined; all other factors are 
equal (except that now I don't have either seq-25 or seq-24).

Indeed, the definition in emacs 24.5 is the following from seq-24.el:
(defun seq-subseq (sequence start &optional end) ... ),
a normal function definition, with a normal, fleshed out body.

In 26.3, it looks like the definition is the following from seq-25.el is 
(or tries to be):
(cl-defgeneric seq-subseq (sequence start &optional end) ... ),
where the definition drops down to (cl-subseq).

If I've understood the documentation of (cl-defgeneric) it should, on 
first invocation of (seq-subseq), grab the right defining file 
(seq-25.elc vs. seq-24.elc). I find it hard to believe that 
(cl-defgeneric) is buggy, because, if it were, then there would be an 
intolerable plethora of such problems,

So, I assume that I've managed to do something wrong. Any thoughts?

Thanks.

-- 
,Doug
d.lewan2000@gmail.com
(908) 720 7908

Anti-intellectualism has been a constant thread winding its way through our political and cultural life,
nurtured by the false notion that democracy means that 'my ignorance is just as good as your knowledge.
     - Isaac Asimov