3-dim. vector if x and < are vectors, a matrix of
3-dim. vectors if x and y are matrices themselves.
Details
Computes the cross (or: vector) product of vectors in 3 dimensions.
In case of matrices it takes the first dimension of length 3 and
computes the cross product between corresponding columns or rows.
The more general cross product of n-1 vectors in n-dimensional
space is realized as crossn.