Strictly speaking, the effective projectivity for a particular edit should be computed based on the intermediate form upon which the edit operates, since the projectivity properties of this form can depend on preceding edits.
the Nat-Log system minimizes the need to compute projectivity in intermediate forms by reordering the edits in an alignment in such a way that effective projectivity can, in most cases, simply be taken from the projectivity marking of P and H performed during the linguistic analysis stage.