How to make some rows in datagrid selected by default?

[kkspeed]

Hello, I'm new to weblocks. Even if with some previous experience with
Django, I find myself totally lost in Weblocks. I've got a few
questions.

1) I have a datagrid created as follows to make all items selected by
default:
(defun make-books-datagrid ()
(make-instance 'datagrid
:name 'books-data-grid
:data-class 'book
:view 'book-table-view
:widget-prefix-fn (lambda (&rest args)
(declare (ignore args))
(with-html (:h1 "Books")))
:allow-select-p t
:selection (cons :all nil)))
in the document, about :selection keywords, it says:
Contains a cons cell that identifies currently selected elements. That
'car' of the cell is either :all or :none. In case of :all, all items
except the ones in the cdr list are selected. In case of :none, none
of the elements except the ones in the cdr list are selected.

but when I've done compiling the form, the page would give me an error
message saying that :all has fallen through the ECASE:
:ALL fell through ECASE expression. Wanted one of (:NONE).

I'm wondering how to make some elements selected by default?

2) Even if finished reading the tutorial of "simple-blog", I'm still a
little bit confused about the parts "layout.lisp/views.lisp/widgets".
What roles do they actually play in weblocks framework? Currently I've
only got the impression that "layout" is responsible for organize the
widgets. views define a way of presenting the data and widgets take
in the data presented by views to form html elements. Does this
understanding make sense?

Any help will be appreciated.