Missing warning when using the MINUS operator

[daniel86]

Hi! I would like to use rasqal in a project, and there are some cases where I need negation. I know about the workaround using OPTIONAL and BOUND (which might be ok for the cases I consider where I know that the object field is unbound before entering the optional).

But first I just tried to use MINUS without being aware it is not supported. The parser supports it, so no syntax error is reported. But it seems then the algebra does not generate a warning here, which I think would be a good idea:

https://github.com/dajobe/rasqal/blob/master/src/rasqal_algebra.c#L1582

In general, it would be great if MINUS could be supported. Is it problematic to do so?

Here is a related ticket in the self-hosted bug tracker: https://bugs.librdf.org/mantis/view.php?id=514