code: private FormattedStringBuilder fillBuffer(int newLength, char c) {
          for (int i = _delegate.length(); i < newLength; i++ ) {
              _delegate.append(c);
          }
          return this;
      }
docs: Attempts to append <tt>c</tt> until the buffer gets the length <tt>
  newLength</tt>. If the buffer is already as long or longer than <tt>
  newLength</tt>, then nothing is appended.
  @param newLength Minimal new length of the returned buffer.
  @param c Character to append.
  @return The updated instance of FormattedStringBuilder.