How to use the bpy_prop_collection find method?

I am trying to get the index of a stroke in a GPencilFrame’s strokes bpy_prop_collection. The bpy_prop_collection has a method called find. The documentation says that it accepts key as a parameter which is a string identifier for the collection member.

How do I get this key in, say, a for loop iteration? When I use the keys() method for a Grease Pencil’s Strokes bpy_prop_collection it returns an empty list. If I cast the collection as a List type, I can use Python’s index method, but I would like to understand find. Thank you!

    for stroke in layer.active_frame.strokes:
        for pt in stroke.points:
            if pt == point: #point is a selected point
                print("keys",layer.active_frame.strokes.keys()) # []
                #print(layer.active_frame.strokes.find(s)) # returns an exception set
                print(list(layer.active_frame.strokes).index(s)) # 0
                return stroke