code: public static void addEquals( DMatrix2 a , DMatrix2 b ) {
          a.a1 += b.a1;
          a.a2 += b.a2;
      }
docs: <p>Performs the following operation:<br>
  <br>
  a = a + b <br>
  a<sub>i</sub> = a<sub>i</sub> + b<sub>i</sub> <br>
  </p>
  
  @param a A Vector. Modified.
  @param b A Vector. Not modified.