code: Paragraph<PS, SEG, S> concatR(Paragraph<PS, SEG, S> that) {
          return this.length() == 0 && that.length() == 0
              ? that
              : concat(that);
      }
docs: Similar to {@link #concat(Paragraph)}, except in case both paragraphs
  are empty, the result's paragraph style will be that of the argument.