theorem
  p1-p2 = |[ p1.1-p2.1, p1.2-p2.2, p1.3-p2.3 ]|
proof
A1:  (-p2).1 = -p2.1 by Th52;
A2:  (-p2).2 = -p2.2 by Th52;
  (-p2).3 = -p2.3 by Th52;
     then p1+-p2 = |[ p1.1+-p2.1, p1.2+-p2.2, p1.3+-p2.3 ]| by A1,A2,Lm2;
     hence thesis;
end;
