sub list_all_even {
   _list_all_even_from(0)
}
sub _list_all_even_from {
   my $from=shift;
   ($from, _list_all_even_from($from+2))
}

##</code><code>##

sub head {
  my ($n,@list)=@_;
  $n ? ($list[0],head($n-1,butfirst(@list))) : ()
}
sub butfirst {
  shift;
  @_
}