it's basically `union <name>([<type>],*)`, i.e. => named sum type implicitly tagged by it's variant types but not "sealed", as in no artificial constraints li

it's basically `union <name>([<type>],*)`, i.e. => named sum type implicitly tagged by it's variant types but not "sealed", as in no artificial constraints like that the variant types need to be defined in the "same place" or "as variant type", they can be arbitrary nameable types