code: public static void addEquals( DMatrix4x4 a , DMatrix4x4 b ) {
          a.a11 += b.a11;
          a.a12 += b.a12;
          a.a13 += b.a13;
          a.a14 += b.a14;
          a.a21 += b.a21;
          a.a22 += b.a22;
          a.a23 += b.a23;
          a.a24 += b.a24;
          a.a31 += b.a31;
          a.a32 += b.a32;
          a.a33 += b.a33;
          a.a34 += b.a34;
          a.a41 += b.a41;
          a.a42 += b.a42;
          a.a43 += b.a43;
          a.a44 += b.a44;
      }
docs: <p>Performs the following operation:<br>
  <br>
  a = a + b <br>
  a<sub>ij</sub> = a<sub>ij</sub> + b<sub>ij</sub> <br>
  </p>
  
  @param a A Matrix. Modified.
  @param b A Matrix. Not modified.