# How to Assign value to a symbolic function?

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?