relevant code:
#composing the types on initialization (from YAML)
$new-model.HOW.add_attribute($new-model, Attribute.new(
:name<@.columns>, :has_accessor(1), :type(Array), :package($new-model.WHAT),
));
#later in BUILD
multi submethod BUILD (:$!driver, :$!db, :$!quote, :$!dbo, :@columns?) {