On second thought, an eval method is probably not the way to go but special-handling of Sum and Piecewise should happen. And perhaps with some care since while I would convert `Sum(x+x0, (i, 1, n))` to `Sum(x, (i, 1, n))` I would not convert `Sum(x, (i, 1, x0))`  to 0 (by ignoring it).