sMarked_sumsum+Marked_+0yxsum(0)0sum(s(x))+(sum(x),s(x))+(x,0)x+(x,s(y))s(+(x,y))sum(0)0sum(s(x))+(sum(x),s(x))+(x,0)x+(x,s(y))s(+(x,y))Marked_sum(s(x))Marked_+(sum(x),s(x))Marked_sum(s(x))Marked_sum(x)Marked_+(x,s(y))Marked_+(x,y)sum(0)0sum(s(x))+(sum(x),s(x))+(x,0)x+(x,s(y))s(+(x,y))Marked_sum(s(x))Marked_sum(x)+sums+sumssums+sum(0)0sum(s(x))+(sum(x),s(x))+(x,0)x+(x,s(y))s(+(x,y))Marked_+(x,s(y))Marked_+(x,y)+sums+sumssums+