code: def build_url (self):
          
          super(MailtoUrl, self).build_url()
          self.addresses = set()
          self.subject = None
          self.parse_addresses()
          if self.addresses:
              for addr in sorted(self.addresses):
                  self.check_email_syntax(addr)
                  if not self.valid:
                      break
          elif not self.subject:
              self.add_warning(_("No mail addresses or email subject found in `%(url)s'.") % \
                  {"url": self.url})
docs: Call super.build_url(), extract list of mail addresses from URL,
          and check their syntax.