using the 'multiple' flag with dfInputSelectGroup

[KasperJanssens]

I can use the multiple flag in the dfInputSelectGroup and it indeed shows a list, in which I can select multiple values, but it seems to be impossible to return multiple values from the form any way I try to the backing record. So maybe it's more of a question of me not getting it than this really being an issue, but dfInputSelectGroup seems to want to be backed by a 'choice' formlet, and all those choice formlets seem to return a single type a from the list of types [a] being passed as possible choices.