Nongeneric null vectors

We consider the generic condition for null directions at a fixed point. A nongeneric vector is one which violates the generic condition: a vectorX is nongeneric ifX^cX^d X[_aR_b]cd[_eX_f] = 0. The presence of null nongeneric vectors at a point can force the curvature tensor to be uniform, i.e., be that of a constant-curvature space; in particular, if there are 11 null nongeneric directions generically situated, then the curvature is uniform. For non-uniform curvature, the locus of nongeneric null directions must obey a cubic relation; examples show that it need not obey a linear relation.