I think it would be best to make the documentation (and variable naming)
for this as similar as possible to that of `gethash', which also has a
default argument. In particular, I think "default" is a better thing to
call it than "sentinel", though it's more important to call it the same
thing in both contexts.

On Fri, Sep 11, 2015 at 1:45 PM, Stefan Monnier <monnier@iro.umontreal.ca>
wrote:

> >     Add seq-find
> >     This function is similar to `seq-some' but returns the found element.
> >     In the cases where nil can be the found element, a sentinel optional
> >     argument can be provided to avoid ambiguities.
>
> This text was not needed here.  It's already present in the code (where
> it belongs).
>
> > +  "Return the first element for which (PRED element) is non-nil in SEQ.
> > +If no element is found, return SENTINEL or nil.
>
>   "return SENTINEL or nil" should be just "return SENTINEL".
>
> Not only it's shorter, but it's also more precise (the current text
> doesn't say when it returns SENTINEL and when it returns nil).
>
> > +Note that `seq-find' has an ambiguity if the found element is nil
> > +and if no SENTINEL is specified, as it cannot be known if an
> > +element was found or not."
>
> It's not just for nil but for the case where the found value is
> identical to SENTINEL.
>
>
>         Stefan
>
>