code: public void mutableSubtract(double c, Matrix B, ExecutorService threadPool)
      {
          mutableAdd(-c, B, threadPool);
      }
docs: Alters the current matrix to store <i>A-c*B</i>
  @param c the scalar constant to multiply <i>B</i> by
  @param B the matrix to subtract from <i>this</i>.
  @param threadPool the source of threads to do computation in parallel