code: public void appendSubstring(StringBuilder appendable, int start, int end) {
  
      appendable.append(this.buffer, this.initialOffset + start, end - start);
    }
docs: This method appends the {@link #substring(int, int) substring} specified by {@code start} and {@code end}
  to the given {@code buffer}. <br>
  This avoids the overhead of creating a new string and copying the char array.
  
  @param appendable is the buffer where to append the substring to.
  @param start the start index, inclusive.
  @param end the end index, exclusive.
  @since 7.5.0