code: def new_section(self, doc, *args, **kwargs):
      
  
      section = Section(doc, *args, **kwargs)
      if section.identifier:
        if section.identifier in self.sections:
          raise ValueError('section identifier {!r} already used'
            .format(section.identifier))
        self.sections[section.identifier] = section
      doc.sections.append(section)
      return section
docs: Create a new section in the specified document. The arguments for this
      method match the parameters for the #Section constructor.
  
      # Raises
      ValueError: If the section identifier is already used.