Sum and Product Macros

There are similarly prod and sum macros:

  pf = prod(i,N,f(i)); // pf = f(0) * f(1) * ... * f(N-1);
  sf =  sum(i,N,f(i)); // sf = f(0) + f(1) + ... + f(N-1);

One often does not need sum because of the summing property of :>_, and the latter yields a more compact block-diagram drawing.