theorem Th28:
  |{ p, q, a * r }| = a * |{ p, q, r }|
  proof
    |{p, q , a * r }| = - |{ p, a * r,q}| by Th24
                     .= - (a * |{p,r,q}|) by Th27
                     .= - (a * (- |{p,q,r}|)) by Th24;
    hence thesis;
  end;
