problems returning all in default argument

Using similarbyterms with patch applied and views 3 (and Drupal 6), I have the following default argument to allow me to embed the views on none node pages -

if (arg(0) == 'node') {
return arg(1);
}
else {return 21;}

'21' is currently a temporary number while I develop the site. I want to change this to 'all' so that on none node pages it essentially displays recent items as opposed to similar items.

With the wildcard correctly set I have tried -

if (arg(0) == 'node') {
return arg(1);
}
else {return all;}

And have tried -

if (arg(0) == 'node') {
return arg(1);
}
else {return 'all';}

Is this not possible with similarbyterms or am I missing something?

Thanks very much!
Rob