theorem
  (p1-p2)`1 = p1`1 - p2`1 & (p1-p2)`2 = p1`2 - p2`2
proof
  p1 - p2 = |[ p1`1 - p2`1, p1`2 - p2`2]| by EUCLID:61;
  hence thesis;
end;
