Add Copyright.

        :type material: string

        :type holder: string

        :type statement: string

        :type url: string

        :type year: int

def add_copyright(
        self,
        material=None,
        holder=None,
        statement=None,
        url=None,
        year=None
    ):
        """Add Copyright.

        :type material: string

        :type holder: string

        :type statement: string

        :type url: string

        :type year: int
        """
        copyright = {}
        for key in ('holder', 'statement', 'url'):
            if locals()[key] is not None:
                copyright[key] = locals()[key]

        if material is not None:
            copyright['material'] = material.lower()

        if year is not None:
            copyright['year'] = int(year)

        self._append_to('copyright', copyright)