theorem Th2:
  (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:55;
  hence thesis;
end;
