code: public static void subtractEquals( 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.