code: public static double[] minusEquals(final double[] v1, final double[] v2) {
      assert v1.length == v2.length : ERR_VEC_DIMENSIONS;
      for(int i = 0; i < v1.length; i++) {
        v1[i] -= v2[i];
      }
      return v1;
    }
docs: Computes component-wise v1 = v1 - v2,
  overwriting the vector v1.
  
  @param v1 vector
  @param v2 another vector
  @return v1 = v1 - v2