what should return unwritable Javascript RDF Datasources for composite-datasources ?

I have done a new Datasource that implements the common interface of 
datasource (GetTarget, Assert, ...). All the function are implemented.

I use this datasource without problems in composite datasources with 
other built-in datasource.

I wanted to do a second datasource in javascript, but this one is not 
writable (as some built-in datasources). But when Assert in a composite 
datasource, my datasource comes first and I don't know what to return 
(exception ? error code ?) to reject properly the assertion and let the 
composite datasource try with other datasource contained...

I'm afraid I'm not so clear... Hopin somebody will understand.
Vaab

PS: I've tryed with throwing arbitrary exception, but this do not work, 
I've outputed the return value of built-in datasources, and they return 
"null" whether they have or have not accepted the assertion.

PS2: I've found references of casting NS_RDF_ASSERTION_REJECTED. There 
are no Components.results.NS_RDF_ASSERTION_REJECTED and I've found that 
it should be :

--- Found in LDAP module for firefox
const NS_ERROR_RDF_BASE = 0x804f0000; // XXX is this right?
const NS_RDF_ASSERTION_REJECTED = NS_ERROR_RDF_BASE + 3;
---

But casting this with a 'throw', or return it doesn't work as expected.

--- Quote from "Modifying RDF Datasource"
Datasources may not accept the change. For instance, when a datasource 
is not writeable or you attempt to add invalid data, the datasource will 
reject the change. In native code, you can detect this as the datasource 
will return an NS_RDF_ASSERTION_REJECTED status code. This status code 
in not an error however.
---

Any hints ?