theorem Th10:
  -p = |[ -p`1, -p`2, -p`3]|
proof
  thus -p = (-1)*p by RLVECT_1:16
    .= |[ (-1)*p`1, (-1)*p`2, (-1)*p`3]| by Th7
    .= |[ -p`1, -p`2, -p`3]|;
end;
