theorem Th25:
  |{ p,q,r }| = - |{ q,p,r }|
  proof
A1: |{ p,q,r }| = p`1 * q`2 * r`3 - p`3*q`2*r`1 - p`1*q`3*r`2 +
      p`2*q`3*r`1 - p`2*q`1*r`3 + p`3*q`1*r`2 by Th23;
    |{ q,p,r }| = q`1 * p`2 * r`3 - q`3*p`2*r`1 - q`1*p`3*r`2 + q`2*p`3*r`1 -
      q`2*p`1*r`3 + q`3*p`1*r`2 by Th23;
    hence thesis by A1;
  end;
