Lars Hellström wrote:

> Not sure what you're asking here. It wouldn't work to reset 
> \ProcessingResult to \l_xparse_arg_toks only at the beginning of parsing 
> the arguments of the command, as any processor can redefine it as a 
> macro and then another can come along that expects it to be a toks 
> register again; it would have to be reset before each processor.

Sorry, that was what I meant to say!
Joseph Wright