code: def add_license(
          self,
          url=None,
          license=None,
          material=None,
          imposing=None
      ):
          
          hep_license = {}
  
          try:
              license_from_url = get_license_from_url(url)
              if license_from_url is not None:
                  license = license_from_url
          except ValueError:
              pass
  
          for key in ('url', 'license', 'material', 'imposing'):
              if locals()[key] is not None:
                  hep_license[key] = locals()[key]
  
          self._append_to('license', hep_license)
docs: Add license.
  
          :param url: url for the description of the license
          :type url: string
  
          :param license: license type
          :type license: string
  
          :param material: material type
          :type material: string
  
          :param imposing: imposing type
          :type imposing: string