theorem Th40:
  for c being Curve of T holds Sum <*c*> = c
  proof
    let c be Curve of T;
    set f = <*c*>;
    len f = 1 by FINSEQ_1:40;
    hence Sum f = (Partial_Sums f).1 by Def14
    .= f.1 by Def13 .= c;
  end;
