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I have the little program:

P = function('P') 
x,y = var('x y')
def S(x): 
    return sum(P(x,y),y,0,1)
S(x)

It returns:

P(x, 1) + P(x, 0)

Now, any idea on how to assign particular values to P(x,y) and return S(x) accordingly?