code: @Override
      public R visitUnknown(TypeMirror t, P p) {
          throw new UnknownTypeException(t, p);
      }
docs: {@inheritDoc}
  
  @implSpec The default implementation of this method in {@code
  AbstractTypeVisitor6} will always throw {@code
  new UnknownTypeException(t, p)}.  This behavior is not required of a
  subclass.
  
  @param t  {@inheritDoc}
  @param p  {@inheritDoc}
  @return a visitor-specified result
  @throws UnknownTypeException
  a visitor implementation may optionally throw this exception