Except in the cases mentioned in 5 [sections 31, 88 and 89], every document to be registered under this Act, whether such registration be compulsory or optional, shall be presented at the proper registration-office,—(a) by some person executing or claiming under the same, or, in the case of a copy of a decree or order, claiming under the decree or order, or (b) by the representive or assign of such a person, or (c) by the agent of such a person, representative or assign, duly authorised by power-ofattorney executed and authenticated in manner hereinafter mentioned