Hi, I'd like to substitute the expression A into B, where B is of the form
where a,b,c,p,q,r, are constants. The problem is: I only want the substitutions to be done outside of the unknown function f. So, if
then I'd like B to become 4*a*u + b*y+c*z + D(f)(p*x+q*y+r*z). This might sound awkward, but in my special case this makes sense. Using subs does not work, because then I get the whole stuff from A also inside the argument of the derivative of the function f, which is not what I want at all. I'm guessing I should use freeze (or frontend), but I have no idea how (mainly because I just discovered they existed). Thanks again!