Use PREFIX in Turtle

[rubensworks]

As suggested by @afs in w3c/sparql-update#31 (comment), we should replace the @prefix syntax in Turtle examples to PREFIX.

[afs]

I've added "discuss-f2f" for a group-wide view on use of PREFIX and @prefix.

[TallTed]

+1 use PREFIX in preference to @prefix

[niklasl]

+1 for using the SPARQL syntax; to aid with e.g. copy-pasting. I'd prefer lower-case prefix in Turtle, since it is intended to be read by humans (and a text/* media-type, not an application/* one). SPARQL is more technical, designed to be more recognizable for SQL users.

[afs]

TriG is "application/trig".
Turtle as "text/turtle" was inherited by RDF 1.1 WG and could not be changed.

[niklasl]

Also use BASE over @base.

@afs True, so it seems I argued for using different forms for those, which would be very confusing. Consistency is paramount, we should use one form for the bulk of the specs, and also make it clear when we introduce the syntax that variations are allowed. And then say something like "in these specs, the PREFIX and BASE forms will be used for consistency".

[afs]

From TPAC 23:
"proposal: use PREFIX in specs over @Prefix across all documents"

(@afs assumes that applies to BASE as well)