docs: public static void changeSign( DMatrix3x3 a )
      {
          a.a11 = -a.a11; a.a12 = -a.a12; a.a13 = -a.a13;
          a.a21 = -a.a21; a.a22 = -a.a22; a.a23 = -a.a23;
          a.a31 = -a.a31; a.a32 = -a.a32; a.a33 = -a.a33;
      }