theorem
  vect(a,a@b) + vect(a,a@b) = vect(a,b)
proof
  vect(a,a@b) + vect(a,a@b) = vect(a,a@b) + vect(a@b,b) by Th27,Th41
    .= vect(a,b) by Th40;
  hence thesis;
end;
