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Manual of the Laws of Naval War. Oxford, 9 August 1913.
Section III : On means of injuring the enemy - Art. 17.
Art. 17. It is also forbidden:
(1) To kill or to wound an enemy who, having laid down his arms or having no longer means of defense, has surrendered at discretion;
(2) To sink a ship which has surrendered, before having taken off the crew;
(3) To declare that no quarter will be given.