docs: public Drawer append(@NonNull Drawer result) {
          if (mUsed) {
              throw new RuntimeException("you must not reuse a DrawerBuilder builder");
          }
          if (mDrawerGravity == null) {
              throw new RuntimeException("please set the gravity for the drawer");
          }
  
          //set that this builder was used. now you have to create a new one
          mUsed = true;
          mAppended = true;
  
          //get the drawer layout from the previous drawer
          mDrawerLayout = result.getDrawerLayout();
  
          // get the slider view
          mSliderLayout = (ScrimInsetsRelativeLayout) mActivity.getLayoutInflater().inflate(R.layout.material_drawer_slider, mDrawerLayout, false);
          mSliderLayout.setBackgroundColor(UIUtils.getThemeColorFromAttrOrRes(mActivity, R.attr.material_drawer_background, R.color.material_drawer_background));
          // get the layout params
          DrawerLayout.LayoutParams params = (DrawerLayout.LayoutParams) mSliderLayout.getLayoutParams();
          // set the gravity of this drawerGravity
          params.gravity = mDrawerGravity;
          // if this is a drawer from the right, change the margins :D
          params = DrawerUtils.processDrawerLayoutParams(this, params);
          // set the new params
          mSliderLayout.setLayoutParams(params);
          //define id for the sliderLayout
          mSliderLayout.setId(R.id.material_drawer_slider_layout);
          // add the slider to the drawer
          mDrawerLayout.addView(mSliderLayout, 1);
  
          //create the content
          createContent();
  
          //create the result object
          Drawer appendedResult = new Drawer(this);
  
          //toggle selection list if we were previously on the account list
          if (mSavedInstance != null && mSavedInstance.getBoolean(Drawer.BUNDLE_DRAWER_CONTENT_SWITCHED_APPENDED, false)) {
              mAccountHeader.toggleSelectionList(mActivity);
          }
  
          //forget the reference to the activity
          mActivity = null;
  
          return appendedResult;
      }