sub add_attributes (MetaClass ^class, Int $defval) { 
    ^class.meta.add_attribute(
         name  => "$.a" ,
         type    => Int,
        default => $defval
    ); 
    ^class.meta.add_attribute(
         name  => "$.c" ,
         type    => Str
    ); 
} 

class George {
     has Int $.d;
     add_attributes(?CLASS, 5); 
}