exceptional case, advance notice is not possible, the requesting State Party shall inform the requested State Party of the disclosure without delay. 20. The requesting State Party may require that the requested State Party keep confidential the fact and substance of the request, except to the extent necessary to execute the request. If the requested State Party cannot comply with the requirement of confidentiality, it shall promptly inform the requesting State Party. 21. Mutual legal assistance may be refused: (a) if the request is not made in conformity with the provisions of this Article; (b) if the requested State Party considers that execution of the request is likely to prejudice its sovereignty, security, public order or other essential interests; (c) if the authorities of the requested State Party would be prohibited by its domestic law from carrying out the action requested with regard to any similar offence, had it been subject to investigation, prosecution or judicial proceedings under their own jurisdiction; (d) if it would be contrary to the legal system of the requested State Party relating to mutual legal assistance for the request to be granted. 22. States Parties may not refuse a request for mutual legal assistance on the sole ground that the offence is also considered to involve fiscal matters. 23. Reasons shall be given for any refusal of mutual legal assistance. 24. The requested State Party shall execute the request for mutual legal assistance as soon as possible and shall take as full account as possible of any deadlines suggested by the requesting State Party and for which reasons are given, preferably in the request. The requested State Party shall respond to reasonable requests by the requesting State Party on progress of its handling of the request. The requesting State Party shall promptly inform the requested State Party when the assistance sought is no longer required. 25. Mutual legal assistance may be postponed by the requested State Party on the ground that it interferes with an ongoing investigation, prosecution or judicial proceeding. 26. Before refusing a request pursuant to paragraph 21 of this Article or postponing its execution pursuant to paragraph 25 of this Article, the requested State Party shall consult with the requesting State Party to consider whether assistance may be granted subject to such terms and conditions as it deems necessary. If the requesting State Party accepts assistance subject to those conditions, it shall comply with the conditions. 27. Without prejudice to the application of paragraph 12 of this Article, a witness, expert or other person who, at the request of the requesting State Party, consents to give evidence in a proceeding or to assist in an investigation, prosecution or judicial proceeding in the territory of the requesting State Party shall not be prosecuted, detained, punished or subjected to any other restriction of his or her personal liberty in that territory in respect of acts, omissions or convictions prior to his or her departure from the territory of the requested State Party. Such safe conduct shall cease when the witness, expert or other person having had, for a period of 15 consecutive days or for any period agreed upon by the States Parties from the date on which he or she has been officially informed that his or her presence is no longer required by the judicial authorities, an opportunity of leaving, has nevertheless remained voluntarily in the territory of the requesting State Party or, having left it, has returned of his or her own free will. 28. The ordinary costs of executing a request shall be borne by the requested State Party, unless otherwise agreed by the States Parties concerned. If expenses of a substantial or extraordinary nature are or will be required to fulfil the request, the States Parties shall consult to determine the terms and conditions under which the request will be executed, as well as the manner in which the costs shall be borne. 29. The requested State Party: (a) shall provide to the requesting State Party copies of government records, documents or information in its possession that under its domestic law are available to the general public; (b) may, at its discretion, provide to the requesting State Party in whole, in part or subject to such conditions as it deems appropriate, copies of any government records, documents or information in its possession that under its domestic law are not available to the general public. 30. States Parties shall consider, as may be necessary, the possibility of concluding bilateral or multilateral agreements or arrangements that would serve the purposes of, give practical effect to or enhance the provisions of this Article. Article 19 Joint investigations States Parties shall consider concluding bilateral or multilateral agreements or arrangements whereby, in relation to matters that are the subject of investigations, prosecutions or judicial proceedings in one or more States, the competent authorities concerned may establish joint investigative bodies. In the absence of such agreements or arrangements, joint investigations may be undertaken by agreement on a case-by-case basis. The States Parties involved shall ensure that the sovereignty of the State Party in whose territory such investigation is to take place is fully respected. Article 20 Special investigative techniques 1. If permitted by the