On Mon, May 3, 2010 at 1:53 PM, Jaime Villate <villate at fe.up.pt> wrote: > f(x,y) := if (y # -x^2) then x*y/(x^2+y) else 0; Better to write if not equal(y, -x^2) then ... here, since "#" is a syntactic comparison only. best Robert Dodier