Thanks for the answer!<div><br></div><div>For now I can go with just having "first", and if needed go for a one/rest approach.</div><div><br></div><div>I couldn't find more efficient ways to take "one" though, and with your comment I'm confident to use it as it is now.</div> <div><br></div><div>[]'s</div><div><br></div><div>Rodolfo Carvalho</div><div><br><div><br></div><div><div class="gmail_quote">On Thu, Sep 8, 2011 at 00:36, Shriram Krishnamurthi <span dir="ltr"><<a href="mailto:sk@cs.brown.edu">sk@cs.brown.edu</a>></span> wrote:<br> <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">This looks like a reasonable way to get an element, but thinking<br> ahead, you will probably want something both like "first" (which you<br> have) and a corresponding "rest".<br> <br> The problem is that which element you get from the "first" operation<br> is nondetermistic in a set. Therefore, you have to either<br> <br> - pass the returned first element to the rest operation, or<br> <br> - accept that<br> (set-union (set (set-item S)) (set-rest S))<br> is not the same as S<br> <br> A better design would be to use a different signature:<br> <br> get-one/rest : (forall (A) (Set A) -> (values A (Set A)))<br> <br> This performs the split "atomically", so it can ensure that the<br> combination of "one" and "rest" is the same as the original set.<br> <font color="#888888"><br> Shriram<br> </font></blockquote></div><br></div></div>