checklist Widget
type ::= (checklist [keyword argument]... type ... )
The type arguments represent each checklist item. The widget's value will be a list containing the values of all checked type arguments. The checklist widget will match a list whose elements all match at least one of the specified type arguments.
The following extra properties are recognized:
:entry-format:greedy:greedy to non-nil, it will allow
the items to come in any sequence. However, if you extract
the value they will be in the sequence given in the
checklist, i.e. the original sequence is forgotten.
:button-args:buttons:children:args