code: public static void subtractEquals( DMatrix2x2 a , DMatrix2x2 b ) {
          a.a11 -= b.a11;
          a.a12 -= b.a12;
          a.a21 -= b.a21;
          a.a22 -= b.a22;
      }
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.