  We propose a notion of weak (n+k,n)-category, which we call
(n+k,n)-Theta-spaces. The (n+k,n)-Theta-spaces are precisely the fibrant
objects of a certain model category structure on the category of presheaves of
simplicial sets on Joyal's category Theta_n. This notion is a generalization of
that of complete Segal spaces (which are precisely the (infty,1)-Theta-spaces).
Our main result is that the above model category is cartesian.
