Help to build a function to approximate a expression when X >> Y

Greetings,

I can not solve this very simple problem.

I do have expressions of the form

(a + b + c + d)*x + (d + e )*y +f

I 'd like to approximate these expressions assuming x >> y in the form

(a + b + c)*x + (d + e )*y +f

but I can not build a function that does this approximation 
automatically (with the expression to be approximated and the two 
variables x and y with x >> y as input arguments)

Could you please help me?

Thanks

Javi