Where, in any trial of an offence punishable under section 7 or under section 11, it is proved that a public servant accused of an offence has accepted or obtained or attempted to obtain for himself, or for any other person, any undue advantage from any person, it shall be presumed, unless the contrary is proved, that he accepted or obtained or attempted to obtain that undue advantage, as a motive or reward under section 7 for performing or to cause performance of a public duty improperly or dishonestly either by himself or by another public servant or, as the case may be, any undue advantage without consideration or for a consideration which he knows to be inadequate under section 11.]