theorem Th41:
  o,p, (o <X> p) <X> (q <X> r) are_LinDep &
  q,r, (o <X> p) <X> (q <X> r) are_LinDep
  proof
    |{o,p, (o <X> p) <X> (q <X> r)}| = 0 &
    |{q,r, (o <X> p) <X> (q <X> r)}| = 0 by Th40;
    hence thesis by Th37;
  end;
