The word Marii (indirect object of the verb nekoupit 'not-to-buy') cannot be reduced as it is 'separated' from its governing verb by the main predicate rozhodl 'decided' (i.e., by the root of the dependency tree) and thus the relation Marii -nekoupit 'to-Mary -not-to buy' is represented by the non-projective edge in the dependency tree.
within projective AR, the shift must be performed to make the reduction possible: → shif t Petr se rozhodl Marii tu knihu nekoupit.