theorem Th74:
  p1<X>p2 = -p2<X>p1
proof
  -p2<X>p1 = |[ (-1)*((p2.2*p1.3)-(p2.3*p1.2)),(-1)*((p2.3*p1.1)-(p2.1*p1.3)),
       (-1)*((p2.1*p1.2)-(p2.2*p1.1)) ]| by Th50
          .= p1<X>p2;
   hence thesis;
end;
