docs: def close( self ):
          """Append a closing tag unless element has only opening tag."""
  
          if self.tag in self.parent.twotags:
              self.parent.content.append( "</%s>" % self.tag )
          elif self.tag in self.parent.onetags:
              raise ClosingError( self.tag )
          elif self.parent.mode == 'strict_html' and self.tag in self.parent.deptags:
              raise DeprecationError( self.tag )