code: protected void processClearBackstack(int flags) {
          if ((flags & CLEAR_BACKSTACK) == CLEAR_BACKSTACK) {
              try {
                  fm.popBackStack(null, FragmentManager.POP_BACK_STACK_INCLUSIVE);
              } catch (IllegalStateException exception) {
                  log.e(TAG, exception.getMessage(), exception);
              }
          }
      }
docs: Process Clear backstack flag.
  <p/>
  FraggleManager will clear the back stack before trying to add the next Fragment if
  {@link #CLEAR_BACKSTACK CLEAR_BACKSTACK} flag is found
  
  @param flags Added flags to the Fragment configuration