Test turtle-syntax-bad-struct-07 - blank nodes in predicates

This test has the triple:

   <http://example/s> _:p <http://example/o> .

I can't see where in the turtle specification is says that you can't have a
blank node as a predicate.

It certainly makes sense to have this constraint.

-- 
--Alex Milowski
"The excellence of grammar as a guide is proportional to the paucity of the
inflexions, i.e. to the degree of analysis effected by the language
considered."

Bertrand Russell in a footnote of Principles of Mathematics