code: public long copyTo(Appendable appendable) throws IOException {
      checkNotNull(appendable);
  
      Closer closer = Closer.create();
      try {
        Reader reader = closer.register(openStream());
        return CharStreams.copy(reader, appendable);
      } catch (Throwable e) {
        throw closer.rethrow(e);
      } finally {
        closer.close();
      }
    }
docs: Appends the contents of this source to the given {@link Appendable} (such as a {@link Writer}).
  Does not close {@code appendable} if it is {@code Closeable}.
  
  @throws IOException if an I/O error occurs in the process of reading from this source or
  writing to {@code appendable}