How can I insert a function in a result in a script?

Hello, I have the final part of a fully working script:

res = (-1)^(n+m)*res

where "res" gives the result:

diff(f(x,y), x) + diff(f(x,y), x,y)

In this result, I'd like to insert the function sin(x), so the output goes straigth to

cos(x)

How can I insert sin(x) after the scirpt has been executed so that it goes in the result and gives a final result?

I tried:

for res subs(f(x,y), exp(-i.*(x+y))) end

but it didnt work