code: public ReadOnlyStyledDocument<PS, SEG, S> build() {
          ensureNotYetCreated();
          if (paragraphList.isEmpty()) {
              throw new IllegalStateException("Cannot build a ReadOnlyStyledDocument with an empty list of paragraphs!");
          }
          alreadyCreated = true;
          return new ReadOnlyStyledDocument<>(paragraphList);
      }
docs: Returns an unmodifiable list of the constructed {@link Paragraph}s and ensures this builder cannot be used again.