def add_section(self, section):
        """Create a new section in the configuration.

        Raise DuplicateSectionError if a section by the specified name
        already exists. Raise ValueError if name is DEFAULT or any of it's
        case-insensitive variants.
        """
        if section.lower() == "default":
            raise ValueError('Invalid section name: %s' % section)

        if section in self._sections:
            raise DuplicateSectionError(section)
        self._sections[section] = self._dict()