theorem Th16:
  Sum sqr <*r1,r2,r3*> = r1^2+r2^2+r3^2
  proof
    thus Sum sqr <*r1,r2,r3*> = Sum <*r1^2, r2^2, r3^2*> by Th15
    .= r1^2+r2^2+r3^2 by RVSUM_1:78;
  end;
