my &helper = -> @prev, @rest { ... }

##</code><code>##

    my sub helper (@prev, @rest) { ... }

##</code><code>##

sub outer(*@vals)
{
   my multi sub helper (@prev) {
      take @prev
   }
   my multi sub helper (@prev, $current, *@rest) {
      $current.map: { helper [@prev, $_], *@rest }
   }
   gather { helper [], *@vals }
}