code: public Matrix4d set3x3(Matrix3dc mat) {
          m00 = mat.m00();
          m01 = mat.m01();
          m02 = mat.m02();
          m10 = mat.m10();
          m11 = mat.m11();
          m12 = mat.m12();
          m20 = mat.m20();
          m21 = mat.m21();
          m22 = mat.m22();
          properties &= ~(PROPERTY_PERSPECTIVE | PROPERTY_IDENTITY | PROPERTY_TRANSLATION | PROPERTY_ORTHONORMAL);
          return this;
      }
docs: Set the upper left 3x3 submatrix of this {@link Matrix4d} to the given {@link Matrix3dc} and don't change the other elements.
  
  @param mat
  the 3x3 matrix
  @return this