code: public Matrix3d zero() {
          m00 = 0.0;
          m01 = 0.0;
          m02 = 0.0;
          m10 = 0.0;
          m11 = 0.0;
          m12 = 0.0;
          m20 = 0.0;
          m21 = 0.0;
          m22 = 0.0;
          return this;
      }
docs: Set all the values within this matrix to 0.
  
  @return this