docs: public static void transpose( DMatrix3x3 m ) {
          double tmp;
          tmp = m.a12; m.a12 = m.a21; m.a21 = tmp;
          tmp = m.a13; m.a13 = m.a31; m.a31 = tmp;
          tmp = m.a23; m.a23 = m.a32; m.a32 = tmp;
      }