Wraps every layout object pointed in `self.slice` under a `LayoutClass` instance with
        `args` and `kwargs` passed, unless layout object's parent is already a subclass of
        `LayoutClass`.

def wrap_once(self, LayoutClass, *args, **kwargs):
        """
        Wraps every layout object pointed in `self.slice` under a `LayoutClass` instance with
        `args` and `kwargs` passed, unless layout object's parent is already a subclass of
        `LayoutClass`.
        """
        def wrap_object_once(layout_object, j):
            if not isinstance(layout_object, LayoutClass):
                layout_object.fields[j] = self.wrapped_object(
                    LayoutClass, layout_object.fields[j], *args, **kwargs
                )

        self.pre_map(wrap_object_once)