theorem
  -|[x1,y1,z1]| = |[ -x1, -y1, -z1]|
proof
A1: |[x1,y1,z1]|`3 = z1;
  |[x1,y1,z1]|`1 = x1 & |[x1,y1,z1]|`2 = y1;
  hence thesis by A1,Th10;
end;
