code: public Matrix3d set(Matrix4fc 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();
          return this;
      }
docs: Set the elements of this matrix to the upper left 3x3 of the given {@link Matrix4fc}.
  
  @param mat
  the {@link Matrix4fc} to copy the values from
  @return this