theorem Th33:
  p1 - p2 = |[ p1`1 - p2`1, p1`2 - p2`2]|
proof
  -p2 = |[ -p2`1, -p2`2]| by Th31;
  then (-p2)`1 = -p2`1 & (-p2)`2 = -p2`2 by FINSEQ_1:44;
  hence p1 - p2 = |[ p1`1 + -p2`1, p1`2 + -p2`2]| by Th27
    .= |[ p1`1 - p2`1, p1`2 - p2`2]|;
end;
