docs: def populate(self, terminal=None, styles=None, default_esc_seq=''):
          '''Get the concrete (including terminal escape sequences)
          representation of this string.
  
          :parameter Terminal terminal: The terminal object to use for turning
              formatting attributes such as :attr:`Root.format.blue` into
              concrete escape sequences.
          :parameter styles: An object from which to look up style definitions,
              which ultimately resolve to escape sequences, but may resolve to
              other intermediaries, including other styles.
          :type styles: StylePlaceholderFactory or dict
          :parameter str default_esc_seq: FIXME
          :returns str: Returns a concrete string, containing all the escape
              sequences required to render the string to the terminal with the
              desired formatting.
  
          .. note::
              All parameters are optional, and will be populated with sensible
              defaults if ommitted. This is to assist testing and experimentation
              with lower-level parts of Junction. If you want to use a
              :class:`StringWithFormatting` object as part of a wider
              application, it is suggested that you always pass all the arguments
              explicitly.
          '''
          terminal = terminal or get_terminal()
          styles = styles or {}
          return ''.join(
              s.populate(terminal, styles, default_esc_seq) for s in
              self._content)