Since 2((y+z)/2)=y+z, we need to take an average and double it. Ah! but we have a linkage for doubling. Indeed, we will get the sum of two complex numbers by constructing a linkage made from two pantographs. The first is not anchored, except that its middle vertex is the middle vertex of the second pantograph, which is anchored to 0 at one side. But worry not about the actual linkage, since we already know we have the function w=2z (as a linkage) and we know we have the function w=(y+z)/2 (as a linkage), then running them serially (i.e., sequentially) we can get sum.

If you were asked to construct a linkage that took 5 inputs and computed the sum of all 5 would you be able to do it?