code: @Override
      public R visitUnknown(Element e, P p) {
          throw new UnknownElementException(e, p);
      }
docs: {@inheritDoc}
  
  @implSpec The default implementation of this method in
  {@code AbstractElementVisitor6} will always throw
  {@code new UnknownElementException(e, p)}.
  This behavior is not required of a subclass.
  
  @param e {@inheritDoc}
  @param p {@inheritDoc}
  @return a visitor-specified result
  @throws UnknownElementException
  a visitor implementation may optionally throw this exception