code: def add_copyright(
          self,
          material=None,
          holder=None,
          statement=None,
          url=None,
          year=None
      ):
          
          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)
docs: Add Copyright.
  
          :type material: string
  
          :type holder: string
  
          :type statement: string
  
          :type url: string
  
          :type year: int