how to apply actions to the tasks of a certain activity.PathPredecessor

I've realized that Task.PathPredecessor Property is Read-only Boolean. but how actions be done to path predecessors

please read the following code :

" through the code I want to change number 1 for  all path predecessors of activity t , activity t is the one with free float"

Sub Macro30()
Dim t As Task
Dim p As Task
For Each t In ActiveProject.Tasks
If t.FreeSlack > 0 Then
For Each p In t.PathPredecessor
p.Number1 = 1
Next p
End If
Next t
End Sub