Re: !=

> to allow an implementation to add handling of types not required by  
> core SPARQL.

Ah, operator extensibility hadn't occurred to me.

>>    * that fn:not is replaced by a SPARQL operator that masks  
>> SPARQL  type errors, treating them as false, but otherwise  
>> behaving as fn:not.
>
> This would lead to a problem with fn:not(fn:not(error)) which is  
> now true.

Good point. Thanks for your helpful response!