code: public static double[] timesMinusTimesEquals(final double[] v1, final double s1, final double[] v2, final double s2) {
      assert v1.length == v2.length : ERR_VEC_DIMENSIONS;
      for(int i = 0; i < v1.length; i++) {
        v1[i] = v1[i] * s1 - v2[i] * s2;
      }
      return v1;
    }
docs: Computes component-wise v1 = v1 * s1 - v2 * s2,
  overwriting the vector v1.
  
  @param v1 vector
  @param s1 scalar for v1
  @param v2 another vector
  @param s2 Scalar
  @return v1 = v1 * s1 - v2 * s2