function vcross = crf( v )
% crf spatial/planar cross-product operator (force).
% crf(v) calculates the 6x6 (or 3x3) matrix such that the expression
% crf(v)*f is the cross product of the motion vector v with the force
% vector f. If length(v)==6 then it is taken to be a spatial vector, and
% the return value is a 6x6 matrix. Otherwise, v is taken to be a planar
% vector, and the return value is 3x3.
vcross = -crm(v)';