Presuppositions of quantified sentences: experimental data

Some theories assume that sentences like (i) with a presupposition trigger in the scope of a quantifier carry an existential presupposition, as in (ii); others assume that they carry a universal presupposition, as in (iii). (i)No student knows that he is lucky. Existential presupposition: At least one student is lucky. Universal presupposition: Every student is lucky.