multi.forwd <- function(x,y){
  return(x*y)
}
multi.bckwd <- function(x, y, dout){
  dx = dout * y
  dy = dout * x
  return(list(dx = dx, dy = dy))
}
 
apple = 100; n.apple = 2; tax = 1.1
apple.pre.tax = multi.forwd(apple, n.apple)
apple.post.tax = multi.forwd(apple.pre.tax, tax)
 
dprice = 1
d.apple.post.tax = multi.bckwd(apple.pre.tax, tax, dprice)
d.apple = multi.bckwd(apple, n.apple, d.apple.post.tax$dx)$dx
d.n.apple = multi.bckwd(apple, n.apple, d.apple.post.tax$dx)$dy