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because from 6|12 we cannot conclude f(12) = 6.
Let f(x) be the y such that y | x.
because we can prove unicity:
Let x/y be the q such that q*y = x.
Since 3*4=12, we may thus conclude 12/4 = 3.
(forall x, y, q: q*y = x => (forall q': q'*y = x => q=q'))