def METHOD_NAME(self):
    super().METHOD_NAME()
    membership = self.cleaned_data["member"].current_membership
    if membership and membership.type == Membership.BENEFACTOR:
        raise ValidationError(_("Benefactors cannot give references."))
    membership = self.cleaned_data["member"].latest_membership
    if (
        membership
        and membership.until
        and membership.until < services.calculate_membership_since()
    ):
        raise ValidationError(
            _(
                "It's not possible to give references for "
                "memberships that start after your own "
                "membership's end."
            )
        )