theorem Th40:
  (PartSums(upper_volume(f,D))).(len D) = upper_sum(f,D)
proof
  len upper_volume(f,D) = len D by Def5;
  then len D in Seg(len upper_volume(f,D)) by FINSEQ_1:3;
  then len D in dom upper_volume(f,D) by FINSEQ_1:def 3;
  then
A1: (PartSums(upper_volume(f,D))).(len D) = Sum(upper_volume(f,D)|(len D ))
  by Def19;
  dom upper_volume(f,D)=Seg len upper_volume(f,D) by FINSEQ_1:def 3;
  then dom upper_volume(f,D)=Seg len D by Def5;
  then upper_volume(f,D)|(Seg len D) = upper_volume(f,D);
  hence thesis by A1,FINSEQ_1:def 16;
end;
