### Piece-wise functions and plotting

Hi,

I have a piece-wise defined function that I want to plot (and potentially do other symbolic stuff with) and I was wondering how to do this. The problem is that I am defining my function as a Python function:

def F(x,y):
if( x <= y ):
return x*y
return x+y

So I am gluing together two pieces, and I would like to be able to do

(x,y) = var('x,y')
contour_plot( F(x,y), (x,0,1), (y,0,1) )

But the problem is that this only plots the second part. This occurs because x <= y evaluates as false (they are variables) and F(x,y) is always evaluated as x+y. In Mathematica there is the **Which** function that works on symbolic expression to make piece-wise definitions. Is there an equivalent in Sage? Is there another way to do this? If I had a function (say, *delta*) that just evaluated as 1 if the symbolic expression was true, and 0 if not, I could craft the function as:

F(x,y) = delta( x<=y ) * (x*y) + delta( x>y ) * (x+y)

But as it is, I think there is no way to do this. Is there?

Thanks a lot for your help,

Edgar