This will work for any ordered signature.  It isn't clear if the set parameter is allowed to be empty, and if so what should be returned.  Below I assume it may be empty, in such case I make the same choice that the designers of `min' made.  This may not work for you.

enum Element {A, B, C, D, E}
fun slub[es : set Element, // or `some Element'?
               e   : Element]:
            lone Element // or `one Element'
{min[{a : es | gt[a,e]}]}

--
Jeremy L. Jacob        Tel: +44-1904-43-2747    Fax: +44-1904-43-2767
Jeremy.Jacob@cs.york.ac.uk    http://www-users.cs.york.ac.uk/~jeremy/
Dept. of Computer Science, The University of York, YORK, YO10 5DD, UK

Thanks a lot