Joss, the list of suggested MUSTs etc is useful although some of these
are contentious

For example 

> I don't know where it is specified that the request may be 
> echoed in SRU 
> but it should better be a must too. The server is allowed to ignored 
> additional request extensions it does not understand - so it should 
> return the query that was actually executed.

I've always wanted this to be a must - however some have argued that
this should only be a should.

Matthew