docs: def siret_validator():
      """Validate a SIRET: check its length (14), its final code, and pass it
      through the Luhn algorithm."""
  
      def _validate_siret(form, field, siret=""):
          """SIRET validator.
  
          A WTForm validator wants a form and a field as parameters. We
          also want to give directly a siret, for a scripting use.
          """
          if field is not None:
              siret = (field.data or "").strip()
  
          if len(siret) != 14:
              msg = _("SIRET must have exactly 14 characters ({count})").format(
                  count=len(siret)
              )
              raise validators.ValidationError(msg)
  
          if not all(("0" <= c <= "9") for c in siret):
              if not siret[-3:] in SIRET_CODES:
                  msg = _(
                      "SIRET looks like special SIRET but geographical "
                      "code seems invalid (%(code)s)",
                      code=siret[-3:],
                  )
                  raise validators.ValidationError(msg)
  
          elif not luhn(siret):
              msg = _("SIRET number is invalid (length is ok: verify numbers)")
              raise validators.ValidationError(msg)
  
      return _validate_siret