Stefan Monnier writes:

> I didn't see much ion their which addresses the issue of setting
> elements in plists (other than plist-put which doesn't seem to be
> significantly better than (push (cons a b) c)).

Multiple pair updates?  As Drew points out, a plist can be treated
like a stack in the same way as an alist.  For lax plists, `nconc'
works, and if you need a true plist, canonicalize-lax-plist is
efficient.

But really, it's a matter of style.